Some chronic HBV carriers have circulating HBV DNA despite the presence of anti-HBeAg antibodies. This observation has recently been related to the presence, in the HBV pre-C region, of a translational stop codon that prevents HBeAg synthesis. In the present study, we analyzed, at the nucleotide level, the pre-C/C region of HBV isolated from the sera of 11 anti-HBe, HBV DNA-positive chronic carriers. Nucleotide sequence analysis of 25 independent clones revealed that the pre-C sequence is highly variable, even among clones derived from the same serum sample. Moreover, our data show that HBeAg synthesis can also be prevented by as yet undescribed T-C substitution at position 1815 that eliminates the start codon of the pre-C transcript. These results suggest that the HBV genome contains segments of high variability that have probably been selected during evolution to favor the segregation of functionally advantaged mutants capable of avoiding host immunity.