In four patients with hemophilia, or von Willebrand's disease, intensive infusion therapy with lyophilized plasma, cryoprecipitate, (Courtland Laboratories), a commercial preparation of fibrinogen and antihemophilic globulin (Fibro-AHF), or antihemophilic globulin (Hemofil) produced hemolysis. The recipients were of blood group A or AB. These materials contained the naturally occurring isoantibodies to A and B erythrocyte antigens. Hemolysis was noted only with intensive replacement therapy. Type-specific cryoprecipitate should be given when prolonged infusion therapy is required following surgical procedures.