SummaryThree cases have been presented who had a circulating antifactor VIII anticoagulant developing spontaneously in non-hemophilic subjects. Following two short courses of azathioprine in one case there were transient incomplete remissions of a degree not seen in the previous 4 months of observation. In the other two cases complete remissions were observed within three weeks of beginning administration of 6-mercaptopurine. In one of these, a brief relapse was retreated successfully. In 4 other cases not given these drugs and in cases reported in the literature, such a rapid remission was not seen to occur spontaneously and happened only infrequently in cases given corticosteroids. On the basis of this experience, we suggest that in the treatment of antifactor VIII, if the disorder shows no improvement with conventional therapy (blood, factor VIII concentrates, and corticosteroids), a trial with immunosuppressive drugs is warranted.