The value of gamma-glutamyl transpeptidase (GGTP) as a general liver function test was assessed in a number of patients. In addition, its value in differentiating between bone and liver disease in patients with unexplained elevation in alkaline phosphatase was studied. This enzyme was measured in 191 patients with hepatobiliary disease and also in 58 patients in whom a high serum alkaline phosphatase (AP) was due to a bony cause. 92: of patients with liver disease had high GGTP values but the test was of little use in differentiating between different types. GGTP is elevated in the same circumstances as AP, but has no advantage over AP as a routine liver function test. Only 2.5% of patients with bone disease had elevated values. GGTP was extremely useful in the differentiation of liver from bone disease, and its greatest use would seem to be in making such a differentiation in patients with an unexplained elevation in AP. The value of gamma-glutamyl transpeptidase (GGTP) as a general liver function test was assessed in a number of patients. In addition, its value in differentiating between bone and liver disease in patients with unexplained elevation in alkaline phosphatase was studied. This enzyme was measured in 191 patients with hepatobiliary disease and also in 58 patients in whom a high serum alkaline phosphatase (AP) was due to a bony cause. 92: of patients with liver disease had high GGTP values but the test was of little use in differentiating between different types. GGTP is elevated in the same circumstances as AP, but has no advantage over AP as a routine liver function test. Only 2.5% of patients with bone disease had elevated values. GGTP was extremely useful in the differentiation of liver from bone disease, and its greatest use would seem to be in making such a differentiation in patients with an unexplained elevation in AP.