and the Edward Daniels Faulkner Arthritis Clinic of the Presbyterian Hospital,

Summary. Cryoproteins observed in patients with systemic lupus erythema-tosus were shown to be of "mixed type, " consisting largely of IgG and IgM. The IgM moiety possessed anti-IgG globulin reactivity and precipitated in the cold after mixing with a source of IgG. The IgM fraction of one cryoprotein precipitated only with the same patient's IgG. Antisera prepared against purified cryoproteins regularly recognized IgM, IgG, the 11 S component of C', and a2-macroglobulin. C'4 (P1E) and C'3 (Pic) were recognized by four and two antisera, respectively. Antisera prepared against two cryoproteins reacted (after absorption) only with the sera of origin. These "unique " anti-gens were associated with IgM, were destroyed by mercaptoethanol treat-ment, and, in one patient, were shown to disappear subsequent to clinical improvement