Single daily injections of 400 mg/kg of chloramphenicol in rabbits combined with sublethal irradiation (600R) are well tolerated for long periods, prolong allograft survival and suppress the establishment of skin reactivity to diphtheria toxoid. Chloramphenicol given in this way alone or irradiation alone has little effect on these reactions. The results of this combined treatment are probably due to anti-inflammatory as well as anti-immune effects.