The present study aims to observe changing patterns of serum antibody to Pneumocystis carinii in normal rats of different ages and in immunosuppressed rats. The serum IgG antibody was observed by immunoblotting with crude antigen of P. carinii which were purified from the lungs of infected rats. The crude antigens separated in SDS-PAGE resolved more than 20 protein bands from 20 to 200 kDa. Of them, 40-45, 50-55, 116 and 200 kDa bands were major antigens of P. carinii. Most of the normal rats of up to 4 weeks had the antibodies reacting the 4 bands, but none of 8-week-old rats revealed the specific antibody. After the rats grew for 40 weeks, all were found to have the antibody in their serum. Same pattern of serum antibody level by age was found in ELISA. When immunosuppressed rats became heavily infected, the antibody in their serum decreased distinctively. The present results suggest that antibodies in normal newborn rats are transferred from their mother and lowered up to 8 weeks. Thereafter, the levels of the antibodies begin to increase by natural exposure to P. carinii. It was also confirmed that the intensity of P. carinii infection is inversely related with levels of serum antibodies.