During the course of hepatitis B virus (HBV) infection, there exists a long period of normal liver function tests with different states of HBeAg/Ab. As the state of HBV in asymptomatic carriers was not well characterized, we quantitatively and qualitatively examined HBV in both HBeAg-positive and anti-HBe-positive asymptomatic carriers. Sera from 10 HBeAg-positive and 27 anti-HBe-positive asymptomatic carriers were analyzed. The amount of HBV DNA was determined by dot-blot hybridization and polymerase chain reaction. The mutations in precore and core regions, spanning 636 nucleotides, of hepatitis B virus were examined by directly sequencing the amplified HBV DNA. HBV DNA was detected in all 10 HBeAg-positive cases, whereas it was found in only 7 of 27 (26%) anti-HBe-positive cases by the nested PCR method. The mean amount of HBV DNA in HBeAg-positive cases was 10(9.1 +/- 0.7) copies/ml, while that in anti-HBe-positive cases was 10(1.0 +/- 1.5) copies/ml. There were no missense mutations in the entire precore and core genes of HBV DNA taken from HBeAg-positive asymptomatic carriers. In contrast, many mutations (mean 9.0 +/- 3.3, range 6-14) were detected in the core gene of seven anti-HBe-positive asymptomatic carriers including two cases with increments of the mutations. Analysis of the precore region revealed three wild-type and four mutant-type (including one coexisting with wild-type) cases. These data suggest that HBV exists in quite different ways in "asymptomatic" carriers; in the HBeAg-positive phase HBV probably coexists with the host and remains as the wild type, whereas in the anti-HBe-positive phase a drastically reduced amount of HBV with many mutations remains, probably as a consequence of the long-lasting interaction with the host. Nevertheless, such small amount of virus could cause fulminant hepatic failure. It is important to make further clinical and virological investigations in order to understand the state of asymptomatic carrier.