AbstractAn immunocytochemical method for identifyingPneumocystis carinii(P. carinii) in formalin fixed, paraffin embedded tissue is described. The procedure utilized a commercially available monoclonal antibody and a sensitive avidin-biotin detection system. The 2G2P. cariniiantibody produced intense labeling of cyst walls, extra-cystic fibrillar material, and trophozoites, even in tissue that had been fixed in formalin for over 2 years. The procedure identified 30 of 30 knownP. cariniiinfected lung tissues. ImmunolabeledP. cariniiorganisms were easily visible under low power magnification. No false positives were observed. No immunoreactivity was observed with a variety of normal tissues and various fungi. The availability of a commercial antibody that reliably and specifically identifiesP. cariniiin histologically processed tissues will benefit pathologists, clinicians, and researchers. (The J Histotechnol13:117, 1990)