Abstract.With complement fixation, Australia antigen was detected in one, and antibody in 6 out of 50 chimpanzees. Seven other apes, and 48 monkeys were negative. All positive chimpanzees clustered in one colony comprising 15 animals, and antibody titres decreased simultaneously over a period of several months. This indicates an infectious pattern of Australia antigenaemia and antibodies in chimpanzees. Antigen and antibodies were undistinguishable from those of man.Australia antigen‐positive serum from a contagious human carrier was injected in 5 chimpanzees. One chimpanzee, with antibody before, but not at the time of exposure, developed circulating antibodies after 13 weeks, followed by antigenaemia, antigen‐antibody complexes, increased transaminase levels and slightly abnormal liver histology. One chimpanzee, without spontaneous antibody, reacted with antibody after 8 weeks, suggesting active infection. No proof of infection was obtained in 3 other anim