Rat Pneumocystis carinii cyst soluble antigen (CSA) was analysed by SDS-PAGE, and more than 20 polypeptide bands were found. The molecular weights of the major bands were > 100kDa, 85-94kDa, 67kDa, 52kDa and 34kDa, respectively. The specific IgG antibodies in the sera from healthy blood donors recognizing the 110kDa, 105kDa, 85kDa, 67kDa, 52kDa and 46kDa polypeptide antigens were identified by immunoblot analysis. The total positive rate of IgG specific antibodies was 56.7% (59/104). The 85kDa polypeptide antigen could also react with the monoclonal antibody derived from human Pneumocystis carinii. The positive rate of the specific anti-85kDa-IgG antibody was 37.5% (39/104), but the specific anti-Pneumocystis carinii IgM antibodies were not detected in all the sera examined. Three of 7 serum samples from the patients with Pneumocystis carinii pneumonia were positive for specific anti-85kDa-IgG antibody (Figs 1-4).