Acridine orange staining of Pneumocystis carinii.

Acridine orange was used to stain smears of mouse lung which contained cyst and trophozoite forms of Pneumocystis carinii. Trophozoite forms stained yellow to orange; however, cyst forms did not stain. Acridine orange is a rapid and sensitive method for demonstrating trophozoites of P. carinii in mouse lung tissue.