AIM--To prepare a monoclonal antibody (EB9) against the trophozoite form of Pneumocystis carinii and to test its efficacy for detecting infection with this organism. METHOD--The sensitivity and specificity of the EB9 antibody were assessed by comparing it with other conventional stains (Papanicolaou, Giemsa and Grocott) and 3F6 antibody in 33 bronchoalveolar lavage specimens from HIV positive patients suspected of having P carinii pneumonia. RESULTS--P carinii infection was detected in 15 of 33 patients. In 14 cases the organism was detected by two or more of the staining methods used; however, EB9 failed to detect infection in two cases which were positive by other staining techniques. In one case P carinii infection was detected by EB9 only. CONCLUSION--The results of this study suggest that P carinii infection in the lung may occur in two forms: the cyst form and the trophozoite form, which may explain the observed variation in response to treatment.