An adverse reaction to the intravenous anaesthetic agent propanidid is described in which the main features were hypotension, facial erythema, and abdominal pain. Changes in serum complement levels and differential white cell counts indicate that this was an immune reaction mediated by the classical complement pathway. The immune reaction apparently involved antibodies other than those of the IgE (reagin) class, and circumstantial evidence suggests that it was specific to propanidid rather than to the entire formulation or to Cremophor EL.