We have used D A-DNA hybridization techniques with a spot test procedure for the detection of hepatitis B virus (HBV) DNA in the serum of 156 HBsAg positive patients with chronic active hepatitis and 75 asymptomatic chronic carriers of the virus. The results were compared with those of HBeAg and anti HBe tests. HBV DNA was detected in the serum of 107 of the 132 (81%) HBeAg positive and 6 of the 24 (25%) anti HBe positive patients with chronic hepatitis. A semi-quantitative estimation of the amount of viral particles showed a marked heterogeneity among the positive sera. HBV DNA was not detected in the serum of 25 patients despite HBeAg positivity: serial samples were available for 13 of these subjects and HBe became undetectable in 9 cases after 6 to 12 months. Among the 75 anti HBe positives asymptomatic carriers HBV DNA was detected in 3 (4%). It appears that HBV DNA detection in the serum is a much more sensitive and direct assay for HBV multiplication than the HBeAg and anti HBe tests.