The nature of kidney lesions in BD IX rats infected with Trypanosoma brucei was investigated. Proteinuria developed and increased up to 236 +/- 35 mg/24 hr at 7 wk after the infection. Antibodies were found to be deposited along the glomerular basement membrane (GBM) predominantly in a linear fashion, which changed to a more granular pattern 7 wk after the infection. At this stage of the disease, electron-dense deposits were found subendothelially along the GBM. In the sera and kidney eluates of diseased rats, anti-GBM antibodies were present. Enzyme-linked immunosorbent assay (ELISA) studies showed antibodies which reacted with GBM components laminin and type IV collagen and not with fibronectin. The antibody specificity was confirmed by using competitive and cross-absorption ELISA techniques, as well as immunoblotting. With the use of indirect immunofluorescence, no common antigenic sites were found on trypanosomes and GBM components. The observed linear immunofluorescence pattern seems to be caused by glomerular binding of antibodies directed against laminin and type IV collagen, which are known to be able to induce renal disease. Subendothelial complex formation in later stages of the disease might result from a molecular rearrangement of GBM components after in situ binding of the antibodies. The formation of auto-antibodies directed against laminin and type IV collagen is probably caused by restricted polyclonal B cell stimulation, a well known feature of trypanosomiasis.