A patient developed serum sickness and acute renal failure following therapy with streptokinase for myocardial ischaemia. There was a previous history of a cellulitic infection of the leg, and antibodies to streptokinase were measurable in a serum sample taken from the patient before therapy. A cryoglobulin was detected at the time of presentation with serum sickness. This contained polyclonal IgG (with anti-streptokinase activity), streptokinase, and C3. Circulating immune complexes were demonstrated by C1q-binding assay. Deposition of C3 was observed in skin and renal biopsies, and bound to erythrocytes. Renal histology, however, showed acute tubular necrosis, with no vasculitis or inflammatory cell infiltrate. This case provides an unusual example of the characterization of an immune complex comprising a specific antibody and an exogenous antigen, and has clinical implications for the use of streptokinase.