Although Pneumocystis carinii pneumonia (PCP) is the most common major opportunistic infection in the acquired immunodeficiency syndrome (AIDS), its immunopathogenesis is not fully understood. It is known that anti-pneumocystis antibodies are present in the sera of individuals with and without PCP. In order to determine whether anti-pneumocystis antibodies are also present in bronchoalveolar lavage fluid (BAL), we looked for them, by immunoreactivity with tissue sections of intra-alveolar P. carinii, in the BAL of (a) HIV-seropositive patients with PCP (n = 18); (b) HIV-seropositive patients without PCP (n = 11); and (c) HIV-seronegative patients with nonpneumocystis lung disease (n = 5). BALs from 19 of 29 HIV-seropositive patients were deficient in at least one isotype (13 with PCP, six without PCP), while only one of five HIV-seronegative patients was deficient. Despite the considerable documentation of atypical presentations of disease caused by P. carinii, little is known concerning the mechanisms involved. To determine whether there is any relationship between BAL anti-pneumocystis antibodies and diverse host responses, we studied antibody binding to P. carinii in different settings. IgG antibodies in BAL bound P. carinii within spleen, liver, skin, and muscle, as well as within pulmonary alveoli and granulomas. However, IgA antibodies in BAL bound intraalveolar and disseminated P. carinii but did not bind to P. carinii within pulmonary granulomas.(ABSTRACT TRUNCATED AT 250 WORDS)