Abstract: BALB/c mice were repeatedly inoculated subcutaneously with purified rat source Pneumocystis carinii (P.c.) cysts.Six weeks later,the spleen cells were fused with NSI myeloma cells,and a mouse hybridoma producing monoclonal antibody against P.c. was establishted and cloned,which was named as 4D7 McAb.Its mean of chromosome was 82.The 4D7 McAb was shown to be IgG1 subclass.It reacted mainly with 54 kDa polypeptide of P.c. in EITB.4D7 McAb could clearly recognize P.c.by immunohistochemical staining in the specimens from confirmed Pneumocystis carinii pneumonia(PCP)patients,and the sensitivity and specificity were both 100% for diagnosis of PCP(11/11).No cross reaction of 4D7 McAb was found with Pneumococcus,Candida albicans and Cryptococcus neoformans,as well as all examined normal human tissues.