Sera from 391 southern African Blacks with hepatocellular carcinoma, matched controls, patients with other malignant tumours, and with various forms of hepatobiliary disease were fractionated by polyacrylamide gradient gel electrophoresis to determine the prevalence of tumour-associated gamma-glutamyl transferase isoenzymes in Black patients with hepatocellular carcinoma. One or more tumour-associated isoenzymes (I', I'' or II') were present in 58.6% of the patients with hepatocellular carcinoma: I' in 54.5%, I'' in 27.1%, and II' in 34%. These isoenzymes were detected in one patient with prostatic cancer, occasionally in patients with acute viral hepatitis, but in no normal individuals. The presence of tumour-associated isoenzymes was not related to patient age, sex or hepatitis-B virus status or to the tumour burden. Isoenzymes were present in 42 percent of hepatocellular carcinoma patients with a normal serum alpha-foetoprotein concentration and in 50% of those with a non-diagnostic value. gamma-glutamyl transferase isoenzymes may be supplementary to alpha-foetoprotein in the diagnosis of hepatocellular carcinoma.