MAb 4F11 recognizes P. carinii derived from multiple different host species. MAbs 4F11 and 1C7, both IgM isotypes derived against P. carinii in mice, were used as probes in an IFA of P. carinii isolates from mice, rats, ferrets, macaques, and humans. Each MAb was used at a concentration of approximately 50 ng/ml.