SummaryAn intensive course of 6‐Mercaptopurine therapy in NZB mice immediately before the overt onset of their autoimmune haemolytic disorder did not delay or modify the subsequent development of antierythrocyte auto‐ antibody or of anaemia.Anaemic mice tolerated the drug poorly. They became more anaemic without lessening of strength of Coombs tests. It is suggested that the drug should be used with caution in anaemic patients with autoimmune disorders.