We evaluated the utility of a monoclonal antibody that recognizes Pneumocystis carinii as a diagnostic tool in specimens of bronchoalveolar lavage fluid and sputum from patients in whom a diagnosis of P carinii pneumonia was being considered. In addition to routine processing for diagnosis by morphologic recognition of P carinii on a Diff-Quik-stained specimen, the specimen was reacted with a monoclonal antibody to P carinii and visualized in an avidin-biotin horseradish peroxidase technique. Of 50 specimens evaluated, there was 94% agreement between results of conventional Diff-Quik staining and immunoperoxidase staining. Two Diff-Quik-positive specimens were negative by immunoperoxidase staining, and one Diff-Quik-negative specimen was positive by immunoperoxidase staining. These discrepancies are most likely attributable to random distribution of P carinii onto smears. The monoclonal antibody used in this study can accurately detect P carinii in induced sputum and bronchoalveolar lavage fluid specimens.