Sera from 600 male homosexual patients were tested for hepatitis B antigen (HBs Ag) and its antibody (HBs Ab). Thirty-one men (5-2%) were positive for HBs Ag. Testing for HBs Ab by immuno-osmoelectrophoresis 33 men (5-5%) were positive. However, sera of 85 patients negative for HBs Ab by routine methods were examined for HBs Ab by radioimmune assay. Thirty (35%) sera were found to be positive. No absolute correlation between the detection of HBs Ag, or previous history of hepatitis, jaundice, or current hepatitis was found. Similarly there was little correlation between presence of HBs Ab and this history. These observations suggest that the male homosexual population represents a pool of individuals within which the hepatitis B virus is readily transmitted, mainly as a subclinical infection although clinical hepatitis does occur in some patients. It is suggested that further work is necessary to determine whether the high antibody rate in male homosexuals is related more to sexual practice than to promiscuity.