A hepatitis B vaccine produced in yeast by recombinant DNA technology was evaluated using 5-micrograms and 10-micrograms doses in a randomized trial lasting 7 months in 110 male armed forces recruits aged 17-19 years. Results were compared to those of an identical trial of a plasma-derived vaccine. No allergic reactions were observed, and the rate of mild side effects was similar to the plasma-derived vaccine. Seroconversion rates in the first month were 60% (33/55) and 67% (37/55) with the 5-micrograms and 10-micrograms doses of the recombinant vaccine, respectively. All participants seroconverted by 3 months, and none lost antibody. These results are very similar to those for plasma-derived vaccine. Comparison of titres of antibody to hepatitis B surface antigen (anti-HBs) showed a slightly higher level with the 10-micrograms than with the 5-micrograms dose of the recombinant vaccine. Geometric mean titres of anti-HBs after the booster dose were similar in the 5-micrograms and 10-micrograms dose recombinant vaccine groups (2,620 and 2,748 IU/l, respectively) and in the 5-micrograms plasma-derived vaccine group (3,591 IU/l) but significantly higher (9,227 IU/l) with the 10-micrograms dose of the plasma-derived vaccine. These results confirm the safety and immunogenicity of the recombinant vaccine, although further study is needed on the duration of immunity.