Pneumocystis carinii pneumonitis in one of the most common life-threatening opportunistic infections in patients with AIDS. The definitive diagnosis of this infection can be established only by demonstration of the organism in clinical specimens. This study was a comparison of methods that provide easy recognition of the organism which is readily available, simple and can be performed rapidly in laboratory-diagnosis. Bronchoalveolar lavage fluids obtained from 35 AIDS patients suspected of having Pneumocystis carinii pneumonitis were examined by three staining methods for the presence of Pneumocystis carinii. With Giemsa stains, P. carinii could be identified in 18 cases (51.4%). Three developmental stages: "cyst", "sporozoite" and "trophozoite" were seen. The contrast of organisms against host cells was not outstanding in these stains. Toluidine blue O stains provided easy recognition of the organisms, with marked contrast between the cysts and host cells. 21 cases (60%) were positive in these stains, but the intracystic structures and trophozoites could not be identified. It was suggested that the clinical specimen should be stained first with toluidine blue O which is more rapid and permits easy recognition of the cyst clusters. If the sporozoites and trophozoites had to be identified, Giemsa stains can be made. In addition, with the methenamine silver nitrate stains, 21 cases (60%) were positive. They revealed the morphology as seen with toluidine blue O but the cost of material may make it unavailable in many laboratories especially with the budgetary restraints of developing countries.