Several naturally occurring variants of immunogenic T cell epitopes were identified within the hepatitis B surface antigen (HBsAg). The effect of these variants on the cellular immune response was studied in individuals vaccinated against HBV. Class-II restricted T-cell responses of 30 vaccinees were analyzed after stimulation of peripheral blood mononuclear cells (PBMCs) with 4 synthetic peptides representing the 4 T-cell epitopes of HBsAg known as of yet. The 2 epitopes P1 (aa 16-33) and P4 (aa 213-226) could be identified as the dominant ones in our vaccinees by proliferation assays and enzyme-linked immunospot assays. Responses to these epitopes were compared with responses to their naturally occurring variants found in HBV isolates of chronic virus carriers. Three of 11 variants of epitope P4 led to a complete loss of T-cell reactivity in 4 of 10 donors, all of whom reacted well to the corresponding wild-type sequence. The remaining 6 donors recognized these variants as well as the vaccine epitope. Similarly, 3 P1-variants of the 12 found induced only a significantly reduced reactivity in 4 of 10 donors, whereas they led to a normal response in the other 6 individuals. Stimulation of T cells also induced the secretion of antibody to HBsAg (anti-HBs) by specific B cells; however, those peptides that failed to activate T cells were also unable to cause any significant anti-HBs production. In conclusion, our results suggest an immune escape of certain mutant strains of HBV in vaccinated individuals could exist at the T-cell level.