The presence of pre-Sl proteins in serum and liver of individuals with acute and chronic hepatitis B virus infection was investigated in Western blots using antibodies against a fusion protein, containing amino acids 20-120 of the pre-S region. Pre-S1 proteins were present in 20 of 38 HBsAg-positive sera. All sera positive for pre-S1 proteins were also positive for hepatitis B virus DNA indicating the presence of hepatitis B virions, and 16 of these sera were also positive for HBeAg. In five sera positive for hepatitis B virus DNA, pre-S1 proteins were not found. In an additional study, pre-S1 proteins could be detected in 4 of 6 patients with acute hepatitis B virus infection during the first 2 weeks after admission to the hospital. The presence of pre-S1 proteins showed a good correlation with the detection of hepatitis B virus DNA. After seroconversion from HBeAg to anti-HBe, both hepatitis B virus DNA and pre-Sl proteins were no longer detectable. Pre-S1 proteins were present in three liver tissue specimens from two patients with acute hepatitis B virus infection and from one patient with cirrhosis of the liver. The proteins were not found in the liver of two HBsAg-positive patients with hepatocellular carcinoma (primary liver carcinoma), negative for HBeAg. Pre-S1 proteins can be detected in serum, positive for hepatitis B virus DNA and in liver tissue of hepatitis B virus-infected individuals. The presence of these proteins appears to correspond with the presence of hepatitis B virus DNA, both markers indicating hepatitis B virus replication.