SUMMARYKidney and skin transplantation sera were studied for rheumatoid-like factors by means of latex test and agglutination test with erythrocytes sensitized by human or rabbit antibodies. Of 23 patients with kidney allografts, 9 developed, after grafting, serum factors combining with human γ-globulin and 6 formed factors combining with both human and rabbit γ-globulin. Of 46 recipients who rejected 3 successive skin allografts, 2 produced anti-human γ-globulin factors and 7 formed anti-rabbit γ-globulin factors. Evidence was presented that the anti-γ-globulin activity of transplantation sera is associated with the γM class of immunoglobulins.The anti-human γ-globulin factor could not be inhibited by the patient's own pretransplantation serum but it was inhibited by pretransplantation sera of some other patients and by some normal human sera. In one instance, the donor's serum was available; it failed to inhibit the activity of the recipient's factor. Anti-rabbit γ-globulin factors had somewhat different serological properties in sera of kidney graft and skin graft recipients.The possible mechanism of formation of the rheumatoid-like factors by allograft recipients was discussed.