Forty healthy adult males were given a 20-micrograms intramuscular dose of either a new recombinant hepatitis B virus (HBV) vaccine produced in mammalian cells (rHBsAg) or the currently licensed HBV vaccine derived from human plasma (HEPTAVAX-B), and booster doses were administered one month and six months later. The percentage of vaccinees who seroconverted on the 10th and 30th days after the first vaccination was higher for the rHBsAg vaccinees than for the recipients of plasma-derived vaccine (rHBsAg: 10% on day 10 and 70% on day 30; HEPTAVAX-B: 0% and 25%, respectively). The only statistically significant difference in titers of antibody to HBV surface antigen was noted 30 days after the first vaccination. Both vaccines induced a shift in antibody from the 19S to the 7S fraction and a specific antibody response to the a determinant. The rHBsAg vaccine was not associated with significant reactogenicity or toxicity and was somewhat more immunogenic than the currently licensed product.