Initial progress has been made toward the development of an experimental vaccine or immunostimulant for Pneumocystis carinii. Antigen derived from cell culture propagated P. carinii proved to be a potent immunogen in the rabbit and antibody thus produced demonstrated identity with intact murine and human lung-derived P. carinii organisms. Reactivity of the antibody with P. carinii soluble antigen in the blood of rats and human subjects with P. carinii pneumonitis (PCP) was demonstrated by the Ouchterlony technique, by counterimmunoelectrophoresis (CIE) and by latex particle agglutination (LPA). Murine-derived P. carinii antigen was utilized in an enzyme-linked immunosorbent assay (ELISA) for anti-P. carinii IgG and IgM produced in immunized rabbits and for human IgG antibody against P. carinii. Preliminary biochemical analysis of whole and solubilized cysts has been carried out, as well as slab gel electrophoresis and immunoblot profiling of solubilized organisms and naturally-occurring P. carinii antigen(s) in the blood of humans and rats with PCP. These studies represent an initial step toward the development of a vaccine or immunostimulant against PCP.