Direct fluorescence monoclonal antibody stain (DFA) was compared prospectively, with calcofluor white (CFW) stain for the diagnosis of Pneumocystis carinii in 163 respiratory specimens from 97 patients. The patient population included persons with HIV infection (58%), bone marrow transplant recipients (10%), immunosuppressed patients owing to chemotherapy (21%) and others (11%). Nineteen specimens including 12 sputa, six bronchoalveolar lavage fluids (BALs) and one induced sputum were positive by DFA. In contrast, only six sputa, and five BALs were positive by CFW. All specimens positive by CFW were also positive by DFA. Of 86 sputa that were negative by either method 29 were followed by more invasive sample collections. Three specimens were followed by induced sputum collection, 18 by BAL, six by lung biopsy, and two by pleural fluid aspiration. All the subsequent induced sputa, pleural fluids, and lung biopsies were negative by both methods. However, four of 18 subsequent BALs (22%) were positive by both methods, provided at least two CFW stained slides were examined per specimen. Except for expectorated sputum, it is concluded that CFW is a rapid and inexpensive test to detect P. carinii in most respiratory specimens.