Pneumocystis carinii pneumonia (PCP) is a frequent cause of pneumonia among human immunodeficiency virus (HIV)-infected patients. Little is known, however, about the role played by humoral immunity to control the infection. This study was undertaken to elucidate the role played by local antibodies. Bronchoalveolar lavage (BAL) fluids from 18 acquired immune deficiency syndrome (AIDS) patients with PCP, 20 HIV-antibody positive patients without PCP, and 20 lung cancer patients were examined for antibodies against P. carinii by the indirect immunofluorescence method. The ratio of albumin concentration in BAL fluid to serum was used to standardize the lavage fluids. Immunoglobulin G (IgG) antibodies against P. carinii occurred less frequently, and immunoglobulin M (IgM) antibodies more frequently, in PCP patients than in other groups. Immunoglobulin A (IgA) antibodies against pneumocysts were found with the same frequency in all three groups, although the median titre was lower among HIV-antibody positive patients without PCP, compared with the other groups. When indexed (antibody titre in BAL fluid x albumin concentration in serum/antibody titre in serum x albumin concentration in BAL fluid) to express locally produced antibodies, IgG indices were significantly higher in HIV-infected patients without PCP, whereas IgM and IgA indices were significantly higher in PCP patients. These findings suggest that the local IgG response is impaired in patients with PCP, whereas the local IgA and, to some extent, the IgM responses are preserved.