We describe a patient with rheumatoid arthritis who developed a sudden, severe thrombocytopenia during the early stages of treatment with sodium aurothiomalate. There were no haemorrhagic manifestations and the thrombocytopenia was rapidly corrected by corticosteroid treatment. The presence of anti-platelet IgG was demonstrated in serial serum specimens by an enzyme-linked immunosorbent assay (ELISA). Although most of this reactivity proved to be directed against allogeneic antigens on donor platelets, IgG reactive with autologous platelets was detected in the first serum sample obtained.