The development of a penicillin-induced haemolytic anaemia was studied in a patient with microangiopathy following an average daily dose of five million units of penicillin G (total dose 46 million units). It has been proposed that the combination of the benzylpenicilloyl hapten with exposed or altered erythrocyte antigens, induces an autoimmune response. In this particular case, it is suggested that erythrocyte membrane damage has predisposed to immune drug-mediated red cell damage.