Monoclonal 3F6 anti-Pneumocystis carinii antibody (MAB-3F6) was used to stain cell blocks from 164 bronchial lavage specimens from patients with the acquired immune deficiency syndrome (AIDS) and AIDS-related complex and compared with slides stained with Grocott's modification of the Gomori methenamine silver stain. Pneumocystis organisms were present in 83 of 164 cases using MAB-3F6 stain, whereas Grocott's modified silver stain demonstrated Pneumocystis organisms in 48. MAB-3F6 demonstrated Pneumocystis organisms in 38 cases with negative silver stains, whereas silver stain identified Pneumocystis organisms in only three MAB-3F6-negative cases. Of 70 patients with clinical Pneumocystis pneumonia at the time of the specimen was obtained, 59 had MAB-3F6-positive specimens, whereas 39 had organisms detected using Grocott's modified silver stain. Of 37 patients without clinically apparent Pneumocystis pneumonia any time in their course, 4 had abundant organisms and 33 had negative stains with MAB-3F6. MAB-3F6 detected Pneumocystis organisms in 22 of 31 cases of Pneumocystis pneumonia that had no organisms identified using Grocott's silver stain (X2 = 5.76, P = 0.016). MAB-3F6 immunochemical staining is a more sensitive method than Grocott's modified silver stain to detect Pneumocystis organisms.