A bleeding diathesis in a case of IgA myeloma was found to be associated with at least two abnormalities in the hemostatic mechanism. Both were related to the presence of the abnormal myeloma protein and improved with plasmapheresis and with response to tryptophane mustard therapy. The first abnormality stems from interference with fibrinogen polymerization and is responsible for prolonging the clotting time, acting as an anticoagulant on addition of the patient’s serum to normal blood, and giving rise to the gelification phenomenon noted on clotting. This abnormality is partially, but not completely, corrected by calcium; it has previously been described in other patients with multiple myeloma. The second abnormality results from coating of collagen (as well as other surfaces) by the abnormal protein and causes the prolonged bleeding time and poor in vivo adhesion of platelets. This abnormality has not been previously described and appears to be partially responsible for the patient’s bleeding.