This paper reports on the diagnostic utility of the 4D7 monoclonal antibody (McAb) and immunohistochemical (IHC) staining to detect P. carinii. Specimens of 30 pathological sections of lung tissues and 1 smear of aspirated bronchial material from confirmed cases, and 12 sputum smears from high risk patients with pneumonia were examined by immunohistochemical method using the 4D7 McAb. The results showed that both the sensitivity and specificity of the 4D7 McAb-IHC staining were 100% (31/31). A comparison between immunohistochemical staining with GMS and TBO staining was made, no difference in sensitivity was found. However, the IHC method gave a clear back ground that is useful for definite diagnosis.