The plasma samples of 11 out of 16 cytotoxicity‐negative hemodialysis patients were found to suppress 43% of 122 mixed leukocyte culture (MLC) combinations. An almost linear relationship was found between the number of transfusions and the blocking activity of the plasma. Plasma from parous women without cytotoxins suppressed 23% of 241 MLC combinations. With plasma from normal controls, only 2.8% of 140 MLC combinations tested were suppressed. The suppressive activity was found to be specific (though somewhat broad) and was absorbable from plasma by the stimulating cells. Inhibition could not be obtained by coating of the responding cells, although a weak degree of suppression was found by precoating the stimulating cells. If suppression of MLC can be explained by the presence of enhancing antibody, active production of such antibodies can be induced by transfusions. Unresponsiveness to incompatible transplants could then be attributed to preconditioning by transfusion