Long-term hepatitis B immune globulin (HBIG) has been shown to reduce hepatitis B virus (HBV) reinfection in patients transplanted for hepatitis B. The aim of this study was to determine the prevalence of HBV S gene mutations in liver transplant recipients who developed recurrent hepatitis B despite HBIG prophylaxis, and to determine if these mutations can revert after withdrawal of HBIG. The entire S gene sequences in pre- and posttransplant sera from 20 patients who developed recurrent hepatitis B despite HBIG prophylaxis were compared. Ten (50%) patients had 18 amino acid substitutions involving the ‘a’ determinant in the posttransplant samples. These mutations were absent in 93% of the pretransplantation clones analyzed. There was a significant correlation between the development of mutations in the ‘a’ determinant region and the duration of HBIG therapy. Most of the mutations result in changes in predicted antigenicity of the S protein. During follow-up, mutations in 14 (78%) of 18 affected codons in the ‘a’ determinant region reverted back to the pretransplantation sequences; only 1 codon had a de novochange after the withdrawal of HBIG. Two control patients who did not receive HBIG had no change in the ‘a’ determinant in their posttransplantation samples. These data support the hypothesis that mutations in the S gene were induced or selected by immune pressure exerted by HBIG. HBV S mutants may play a role in HBV reinfection in liver transplant recipients who received HBIG prophylaxis.