diff --git a/discovery/targetgroup/targetgroup_test.go b/discovery/targetgroup/targetgroup_test.go
index ceb6bf9b9..c4a588e0f 100644
--- a/discovery/targetgroup/targetgroup_test.go
+++ b/discovery/targetgroup/targetgroup_test.go
@@ -17,18 +17,29 @@ import (
 	"errors"
 	"testing"
 
-	"github.com/prometheus/prometheus/util/testutil"
+	"github.com/prometheus/common/model"
 	"gopkg.in/yaml.v2"
+
+	"github.com/prometheus/prometheus/util/testutil"
 )
 
 func TestTargetGroupStrictJsonUnmarshal(t *testing.T) {
 	tests := []struct {
 		json          string
 		expectedReply error
+		expectedGroup Group
 	}{
 		{
 			json: `	{"labels": {},"targets": []}`,
 			expectedReply: nil,
+			expectedGroup: Group{Targets: []model.LabelSet{}, Labels: model.LabelSet{}},
+		},
+		{
+			json: `	{"labels": {"my":"label"},"targets": ["localhost:9090","localhost:9091"]}`,
+			expectedReply: nil,
+			expectedGroup: Group{Targets: []model.LabelSet{
+				model.LabelSet{"__address__": "localhost:9090"},
+				model.LabelSet{"__address__": "localhost:9091"}}, Labels: model.LabelSet{"my": "label"}},
 		},
 		{
 			json: `	{"label": {},"targets": []}`,
@@ -39,15 +50,54 @@ func TestTargetGroupStrictJsonUnmarshal(t *testing.T) {
 			expectedReply: errors.New("json: unknown field \"target\""),
 		},
 	}
-	tg := Group{}
 
 	for _, test := range tests {
+		tg := Group{}
 		actual := tg.UnmarshalJSON([]byte(test.json))
 		testutil.Equals(t, test.expectedReply, actual)
+		testutil.Equals(t, test.expectedGroup, tg)
 	}
 
 }
 
+func TestTargetGroupYamlMarshal(t *testing.T) {
+	marshal := func(g interface{}) []byte {
+		d, err := yaml.Marshal(g)
+		if err != nil {
+			panic(err)
+		}
+		return d
+	}
+
+	tests := []struct {
+		expectedYaml  string
+		expectetedErr error
+		group         Group
+	}{
+		{
+			// labels should be omitted if empty.
+			group:         Group{},
+			expectedYaml:  "targets: []\n",
+			expectetedErr: nil,
+		},
+		{
+			// targets only exposes addresses.
+			group: Group{Targets: []model.LabelSet{
+				model.LabelSet{"__address__": "localhost:9090"},
+				model.LabelSet{"__address__": "localhost:9091"}},
+				Labels: model.LabelSet{"foo": "bar", "bar": "baz"}},
+			expectedYaml:  "targets:\n- localhost:9090\n- localhost:9091\nlabels:\n  bar: baz\n  foo: bar\n",
+			expectetedErr: nil,
+		},
+	}
+
+	for _, test := range tests {
+		actual, err := test.group.MarshalYAML()
+		testutil.Equals(t, test.expectetedErr, err)
+		testutil.Equals(t, test.expectedYaml, string(marshal(actual)))
+	}
+}
+
 func TestTargetGroupYamlUnmarshal(t *testing.T) {
 	unmarshal := func(d []byte) func(interface{}) error {
 		return func(o interface{}) error {
@@ -55,28 +105,28 @@ func TestTargetGroupYamlUnmarshal(t *testing.T) {
 		}
 	}
 	tests := []struct {
-		yaml                    string
-		expectedNumberOfTargets int
-		expectedNumberOfLabels  int
-		expectedReply           error
+		yaml          string
+		expectedGroup Group
+		expectedReply error
 	}{
 		{
-			yaml:                    "labels:\ntargets:\n",
-			expectedNumberOfTargets: 0,
-			expectedNumberOfLabels:  0,
-			expectedReply:           nil,
+			// empty target group.
+			yaml:          "labels:\ntargets:\n",
+			expectedGroup: Group{Targets: []model.LabelSet{}},
+			expectedReply: nil,
 		},
 		{
-			yaml:                    "labels:\n  my:  label\ntargets:\n  ['localhost:9090', 'localhost:9191']",
-			expectedNumberOfTargets: 2,
-			expectedNumberOfLabels:  1,
-			expectedReply:           nil,
+			// brackets syntax.
+			yaml:          "labels:\n  my:  label\ntargets:\n  ['localhost:9090', 'localhost:9191']",
+			expectedReply: nil,
+			expectedGroup: Group{Targets: []model.LabelSet{
+				model.LabelSet{"__address__": "localhost:9090"},
+				model.LabelSet{"__address__": "localhost:9191"}}, Labels: model.LabelSet{"my": "label"}},
 		},
 		{
-			yaml:                    "labels:\ntargets:\n  'localhost:9090'",
-			expectedNumberOfTargets: 0,
-			expectedNumberOfLabels:  0,
-			expectedReply:           &yaml.TypeError{Errors: []string{"line 3: cannot unmarshal !!str `localho...` into []string"}},
+			// incorrect syntax.
+			yaml:          "labels:\ntargets:\n  'localhost:9090'",
+			expectedReply: &yaml.TypeError{Errors: []string{"line 3: cannot unmarshal !!str `localho...` into []string"}},
 		},
 	}
 
@@ -84,8 +134,24 @@ func TestTargetGroupYamlUnmarshal(t *testing.T) {
 		tg := Group{}
 		actual := tg.UnmarshalYAML(unmarshal([]byte(test.yaml)))
 		testutil.Equals(t, test.expectedReply, actual)
-		testutil.Equals(t, test.expectedNumberOfTargets, len(tg.Targets))
-		testutil.Equals(t, test.expectedNumberOfLabels, len(tg.Labels))
+		testutil.Equals(t, test.expectedGroup, tg)
 	}
 
 }
+
+func TestString(t *testing.T) {
+	// String() should return only the source, regardless of other attributes.
+	group1 :=
+		Group{Targets: []model.LabelSet{
+			model.LabelSet{"__address__": "localhost:9090"},
+			model.LabelSet{"__address__": "localhost:9091"}},
+			Source: "<source>",
+			Labels: model.LabelSet{"foo": "bar", "bar": "baz"}}
+	group2 :=
+		Group{Targets: []model.LabelSet{},
+			Source: "<source>",
+			Labels: model.LabelSet{}}
+	testutil.Equals(t, "<source>", group1.String())
+	testutil.Equals(t, "<source>", group2.String())
+	testutil.Equals(t, group1.String(), group2.String())
+}