Hepatitis B surface antigen (HBsAg) and antibody (antiHBs) were determined in 556 narcotic addicts entering a randomized prospective study of alcoholism in patients on methadone maintenance. Only 37% of patients were either negative for HBsAg or without detectable titers of antiHBs. No difference in reactivity existed between alcoholic as compared to nonalcoholic patients and the use of street heroin as compared to use of methadone upon entry to the study. At least a one year follow-up was obtained in 229 (41%) patients with 25 of 88 (28%) patients initially without titers to antiHBs developing detectable titers. No relationship existed between development of antiHBs titers and elevation of liver function tests or the subsequent use of parenteral heroin. These findings suggest that the development of titers to antiHBs in persons on methadone maintenance should not be used as an indication of parenteral drug use.