Precore defective HBV mutants may gradually prevail because of immune selection and explain spontaneous seroconversion from HBeAg to anti-HBe in HBV carriers. We have analyzed whether the presence of precore HBV mutants is a determinant of responsiveness to interferon-α therapy. Fifteen carriers (nine responders and six nonresponders)who were treated with interferon-α were examined. Serum samples were collected before and after therapy. After extraction of DNA, the precore region was amplified by the polymerase chain reaction, and the product was identified by gel electrophoresis and ethidium bromide staining and then Southern blotting and molecular hybridization. The amplified products in all patients were asymmetrically amplified by a modified polymerase chain reaction, and the precore region was directly sequenced. All patients were HBV DNA positive initially. Circulating HBeAg-negative mutants were not identified before treatment in either responders or nonresponders. All nine responders were negative for HBV DNA in serum by dot blot during or after treatment, but seven remained positive by polymerase chain amplification and Southern-blot hybridization. All of the nonresponders remained positive for HBV DNA by dot blot. A silent mutation involving the substitution of an A for G at position 1888 was found in seven carriers; however, no HBeAg-negative mutants were detected in the follow-up of either responders or nonresponders to interferon-α. These results suggest that HBV DNA is detectable by polymerase chain reaction for a period after clearance of HBsAg. A predominance of HBeAg-negative HBV virions cannot be considered a pivotal determinant of response to treatment. The role of other mutations in the precore region in determining response to therapy is uncertain. (HEPATOLOGY 1992;15:1002–1006).