Two HLA class II haplotypes, HLA-[DQB1*0602; DQA1*0102; DR15] and HLA-[DQB1*0603; DQA1*0103; DRB1*1301] were found to be less common in 52 nonresponders compared with 68 responders, P<0.025 and P<0.05 respectively, after vaccination with hepatitis B surface antigen (HBsAg). Another haplotype, HLA-[DQB1*0604; DQA1*0102; DRB1*1302], had a significantly higher frequency in the nonresponders (P<0.005). The nonresponders and responders were nonsmoking, healthy individuals with an antibody concentration of <10 IU/l and >100 IU/l respectively. The three haplotypes comprise either of three different DQB1*06 subtypes. Two of the seven amino acids that differ between the two responder alleles DQB1*0602 and *0603 and the nonresponder allele *0604 are located in the peptide-binding groove of the DQB1 molecule. In addition to this finding, amino acid 86 in the DRB1 molecule seems to determine the response against HBsAg. DRB1*1301 and DR15 in the responder haplotypes have a Val at this position while the nonresponder haplotype has a Gly. These results suggest a role for both the DQB1*06 alleles and the DRB1 alleles *1301, *1302 and DR15 to direct either a response or a nonresponse against HBsAg. Sixteen HLA class II genotypes were found to be shared by 25 nonresponders and 32 responders. This finding of HLA-identical nonresponders and responders indicates an influence of other genetic factors in addition to the HLA system in the response to HBsAg.