Chimpanzees were inoculated with human plasmas of known infectivity containing hepatitis B surface antigen (HBS Ag) of both d and y subspecificities in an attempt to examine the subspecificity characteristics of chimpanzee antibody responses. Our findings show that chimpanzees inoculated with HBS Ag of known subtype develop transient or persistent infection and acquire antigen and/or antibody of corresponding subspecificity. Antigenic subspecificity in relation to cross-protection was examined. Reinfection with a different viral subtype was not demonstrated and appears unlikely in adult chimpanzees which are not immunosuppressed or suffering from other chronic diseases.