A patient with an extramedullary plasmacytoma was diagnosed as having multiple myeloma associated with type I (κ) IgA cryogelglobulin. Treatment of the patient with cyclophosphamide resulted in a marked decrease in the serum IgA level with a concomitant increase in serum gelling time. Physical studies of the whole serum and the partially purified cryogelglobulin showed it had a sedimentation coefficient of 8.8S and an approximate isoelectric pH of 5.5. Alteration of disulfide bonds without alkylation greatly increased the gelling time of the cryogelglobulin.