Immunodetection of Pneumocystis carinii, based on immunofluorescence and use of a monoclonal antibody specific for an antigen located within the cyst wall and detectable after trypsin digestion only, was compared with a methenamine silver stain in 553 bronchoalveolar lavage specimens from immunosuppressed patients. P. carinii was found by immunofluorescence in 72 (86%) and by silver stain in 68 (81%) of the total of 84 positive samples detected by either or both of these methods.