The rapid identification of Pneumocystis carinii in respiratory specimens is critical for the optimal care of a variety of immunosuppressed patients, including in particular those with AIDS, who are especially subject to infection with this organism. A variety of staining techniques are already available, including toluidine blue and monoclonal antibody stains. The currently available stains are all moderately time-consuming to perform. A particular calcoflour white stain (Fungi Flour, manufactured by Polysciences, Inc., Warrington, PA) has been found to stain P. carinii cysts. When evaluated in parallel with the toluidine blue and monoclonal antibody stains, this stain appeared to have a slightly lower sensitivity and specificity, especially for sputum specimens.