We report on a patient who developed acute intravascular immune haemolysis while receiving carbimazole. Serological studies revealed a strongly (3+) and a discretely positive (1+) direct antiglobulin test due to C3d and IgG respectively, and a very weak IgG autoantibody in the eluates. Serum from the patient contained specific carbimazole-dependent red blood cell (RBC) antibodies which reacted with all normal human RBC in the presence of free carbimazole as well as with RBC coated with the drug either in vitro or in vivo, although carbimazole itself is not detectable in plasma after oral administration. The results provide direct evidence for the sequence of the drug-RBC-antibody interaction and show that the RBC (and not plasma proteins) function as 'carrier-like' macromolecules in the immune response.