Autoantigens in canine autoimmune haemolytic anaemia (AIHA) were identified by immunoprecipitation using autoantibody eluted from the erythrocytes of affected dogs. At least three patterns of precipitated antigen were identified in six cases of AIHA. The most commonly precipitated antigen pattern was a combination of 42-kD and 29-kD peptides, associated with up to three other membrane components. These autoantigens may be canine glycophorins, which are of similar molecular mass, or may possibly represent an equivalent of the human Rhesus complex. An autoantigen identical in molecular mass to band 3, the erythrocyte anion channel protein, was precipitated in one case of AIHA, and unknown peptides of 37 kD and 100 kD were isolated by autoantibody from another dog. In one case, no antigens were precipitated by the eluted antibody, indicating that the autoantibody may have bound a non-protein membrane component such as phospholipid. Overall it is considered that the different patterns observed may reflect differences in the aetiology of the condition. In other studies, sera from dogs with AIHA failed to immunoprecipitate autoantigens, but were shown by immunoblotting to contain autoantibodies to proteins of the erythrocyte cytoskeleton. Such autoantibodies were also demonstrated in normal canine sera and it is suggested that they are unlikely to play a role in the pathogenesis of AIHA, but may be part of a normal clearance mechanism for damaged red blood cells.