The seroprevalence of hepatitis B virus (HBV) markers and antibody to the delta agent (anti-delta) was determined for 112 parenteral drug abusers entering a methadone maintenance program. Markers of HBV infection were found in 87.5 percent of the group, and seropositivity was significantly associated with duration of drug abuse (p = 0.02). Antibody to the hepatitis B core antigen (anti-HBc) was present in all seropositive subjects; three (2.7 percent) were hepatitis B surface antigen-positive, and 16 (14.2 percent) had only anti-HBc. Five (10.6 percent) of 47 subjects with HBV markers had anti-delta. Anti-delta was more common in subjects who reported multiple symptomatic episodes of hepatitis (p = 0.02) and fewer than three daily drug injections (p = 0.05). Ten susceptible subjects received hepatitis B vaccine, and seroconversion rates at one, three, and six months were 20.0, 88.8, and 100 percent, respectively. The data indicate that hepatitis B vaccine is immunogenic in this population, and that anti-HBc is the optimal prevaccination screening test. Recent outbreaks of fulminant HBV and delta co-infection among drug abusers emphasize the need for early immunization in this group.