The pre-S2 protein of the hepatitis B virus envelope evokes anti-pre-S2 antibodies and enhances the anti-HBs response after vaccination in mice. In order to evaluate the immunogenicity of the pre-S2 Ag in man, 102 HBsAg, anti-HBs, anti-HBc-negative haemodialysis patients were vaccinated at random according to one of four vaccination schedules: 5 micrograms plasma vaccine Pasteur (containing 1% pre-S2) or 20 micrograms plasma vaccine MSD (no pre-S2) at 0, 1, 2, 4, 6 and 12 months; or 10 micrograms Pasteur or 40 micrograms MSD at 0, 1, 2 and 6 months. Anti-HBs levels were measured by RIA and expressed in IU l-1; anti-pre-S2 response was evaluated by both EIA and Western blot analysis. Eighty-four per cent (95% confidence interval: 75-93) of the patients exhibited an anti-HBs response of 2 IU l-1 or more and 71% (95% confidence interval: 61-81) reached an anti-HBs level of at least 10 IU l-1 within 13 months of the start of vaccination. Anti-HBs response correlated with age (the response rate decreased with increasing age) but not with either the type of vaccine or dosage. An anti-pre-S2 response was observed in 16% (EIA) and 46% (Western blot) of the patients immunized with the Pasteur vaccine and none (EIA, Western blot) of the MSD group. The level of anti-pre-S2 antibodies correlated with high anti-HBs titres; an anti-pre-S2 response occurred in only one anti-HBs-negative patient.(ABSTRACT TRUNCATED AT 250 WORDS)