Antibody responses to a major purified human Pneumocystis carinii surface antigen (gp95) were determined by ELISA in human immunodeficiencyvirus (HIV)-infected patients. Serum IgG directed against gp95 was measured in 129 consecutive HIV-infected patients who underwent bronchoscopy for evaluation of pulmonary symptoms. Significantly more patients with P. carinii pneumonia (PCP) had detectable antibodies compared with HIV-infected patients without PCP and with HIV-negative controls (50 [66%]of 76 vs. 18 [34%]of 53 and 7 [35%]of 20, respectively;P < .001), and the levelof antibody response was higher (mean optical density ratio: 0.6 vs. 0.23 and 0.2, respectively;P < .01). Changes in antibody response were investigated in 78 patients for whom serial serum samples taken around the time of bronchoscopy were available. Of the 47 patients with verified PCP, 20 (43%) mounted an antibody response, compared with only 1 (3%) of 31 patients without PCP (P < .001). This patient had PCP on the basis of clinical criteria, including response to therapy. Thus, despite severeimmunosuppression, a proportion of HIV-infected patients with PCP can mount a specific IgG-mediated antibody response to P. carinii.