Owing to recent findings of certain unusual sex steroid binding in liver disease--particularly an allosteric biphasic pattern (pattern A) unique to the serum of patients with hepatocellular carcinoma--the serum binding characteristics for 5 alpha-dihydrotestosterone were examined in serum samples from six patients with primary biliary cirrhosis who had developed hepatocellular carcinoma. In all serum samples taken after the development of tumour pattern A binding only was obtained, and in four cases in which earlier samples were also examined there was a transformation from the normal, non-specific binding pattern, or an allosteric plateau pattern seen in non-malignant liver disease (designated D and C respectively), to pattern A coincident with the rise in serum alpha fetoprotein. In one patient chemotherapy leading to a fall in alpha fetoprotein abolished pattern A binding, showing further its close association with tumour growth. The value of pattern A binding as a tumour marker in hepatocellular carcinoma warrants further study.