A formalin-inactivated aluminium hydroxide adsorbed hepatitis A vaccine was evaluated in a dose-response study on 195 healthy male adults (age range: 18-31 years) in two French hospitals (Lyon, Rouen). Four doses (20, 40, 80, 160 RIA antigen units) were administered intramuscularly (i.m.) in two injections over a 6-month period. At the time of the first vaccine injection, 32 subjects (16.4%) were found positive (> 20 mIU ml-1) for HAV antibody (total Ig RIA HAVAB assay, Abbott Laboratories) and were excluded from the analysis of immunogenicity criteria. Fourteen days after the first vaccine injection, 78.1% (95% confidence interval (CI): 62-90) of seronegative subjects who received the 160 RIA antigen unit dose seroconverted with a geometric mean titre (GMT) of 43 mIU ml-1 (95% CI: 33-56). Seroconversion was 100% (95% CI: 91-100) at 1 month with a GMT of 95 mIU ml-1 (95% CI: 79-112). Statistical analysis revealed a significant dose-related effect (p < 0.0001) on GMT by multivariate regression analysis of the results after the first injection. Biological safety was evaluated and alanine aminotransferase and aspartate aminotransferase levels were similar prior to and 14 days after the first injection in the four groups. Reactions after injection were similar in the four dosage groups: 6.2% of subjects reported immediate reactions after first vaccination (feeling sick, spontaneous pain, headache), 8.9% reported local reactions at the site of injection (spontaneous pain, haematoma, local adenopathy) and 13.5% reported general reactions ('flu-like' syndrome, gastrointestinal tract disorders, fatigue, headache).(ABSTRACT TRUNCATED AT 250 WORDS)