Immunotropic properties of hydrazide cysteine (HD) were studied in vivo and in vitro using mouse thymocytes, splenocytes and peripheral non-B lymphocytes (PBL). HD in the doses used, was not cytotoxic, increased resistance to HC of all cells examined and caused an elevation of ARFC levels in thymocytes and splenocytes being inhibitory in relation to PBL. HD, after in vitro contact with PBL induced differentiation of precursor cells into Lyt 2+ lymphocytes.