Arginine was found to be completely depleted from cell-free supernates of mixed leukocyte cultures suppressed by the addition of excess macrophages. Partial reversal of macrophage-mediated suppression was accomplished by daily addition of a cocktail containing arginine and nonamino acid nutrients. Complete reversal of the suppression was accomplished by the addition of arginine and glucose to the medium and the nonadherent cells after their separation from the adherent macrophages. A marked increase in the enzyme arginase was found in macrophages that had been cultured 24 h in vitro in Eagle's minimum essential medium plus 10% fetal calf serum, in peritoneal cells activated by prior injection of thioglycollate, and in one spleen activated by a graft vs. host reaction.

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