The study concerns the effect of cysteine derivatives and its hydrazides on cell mediated immunity. Cysteine derivatives appeared to exert marked suppressive influence on development and course of post-adjuvant polyarthritis in rats. L-cysteine hydrazides prolonged skin allograft survival time in rats to higher degree than cysteine. The results of in vitro experiments point to the inhibition of the non-stimulated and stimulated PHA incorporation of 3H-thymidine by lymphocytes. It was observed that cysteine derivatives examined, and mainly its hydrazides, exert stronger than cysteine suppressive effect on cell mediated immune response.