A 35-year-old female hepatitis B virus carrier chimpanzee was infused with one dose of a mixture of human monoclonal antibodies 9H9 and 4-7B (antibodies against hepatitis B virus surface antigen; HBsAg). Blood samples were taken before and up to 3 weeks after infusion. HBsAg and antibodies against HBsAg (anti-HBs) were quantified by radioimmunoassay and enzyme immunoassay. Free anti-HBs was never detected. Thirty min after the start of the infusion the HBsAg level was minimal with maximum loading of the chimpanzee HBsAg with human immunoglobulin. HBsAg complexes could be dissociated by acid treatment. The HBsAg level was completely restored on day 7. Similar results were obtained for the preS1-containing particles that may represent the infectious viral particles in the chimpanzee serum. A mouse monoclonal anti-HBs (HBs.OT40) was found to compete with 9H9 in artificial immune complexes with the pre-treatment HBsAg from the chimpanzee. Used as a conjugate, HBs.OT40 yielded a maximum decrease in the signal in the 30 min sample compared to non-competing anti-HBs conjugates. This indicates binding of HBsAg with 9H9 in the circulation of the chimpanzee. Immune-complexed 4-7B could not be detected by its corresponding 4-7B peptide conjugate, probably due to its low concentration in the complexes. It is concluded that human monoclonal anti-HBs can effectively reduce the level of HBsAg in serum from this chronic carrier. Monoclonals 9H9 and 4-7B may complement each other due to their different mechanisms of inactivation, probably with higher efficiency than that monitored by our HBsAg screening assays.