The platelet aggregation test (PLA) can be used as an indicator of Australia antigen or antibody in the serum.1,2 The reaction of Au antigen-Au antibody complexes with human platelets changes the pattern of sedimentation of platelets on microplates.2 This method has also been used to detect other antibodies and antigens.3 When PLA was used to study hepatitis sera, it was soon found that many of these sera reacted directly with platelets without any added antibody or antigen. Earlier studies show that this can be a sign of the presence of immune complexes in the blood.4 The possibility of immune complexes in severe hepatitis cases has recently been under special consideration.5 This paper presents the results of 93 cases of different forms of hepatitis, 49 other diseases, and 99 blood donors. Special consideration is given to the "direct" reactivity of sera. The detection of the