Two human monoclonal antibodies with anti-hepatitis B activity were investigated separately and as a mixture by means of an “inhibition in solution” assay. With this assay the capacity of anti-HBs antibodies to inhibit the binding of hepatitis B surface antigen (HBsAg) with solid-phase anti-HBs (Ausria II, Abbott Laboratories) was studied. Both HBsAg of different subtypes and purified Dane particles were used. One of the monoclonal antibodies (9H9) was directed against a conformational epitope (anti-“a” like) but induced incomplete inhibition (80% to 90%) of all HBsAg subtypes; the other (4–7B), active against a linear epitope, caused full inhibition of all HBsAg subtypes except one (HBsAg/adw4). In vitro, a mixture of these two monoclonal antibodies was active against HBsAg from liver transplant recipients, including those with suspected variant viruses. (HEPATOLOGY 1995; 22:1078–1083.).