Thymus cells from untreated or hydrocortisone-treated mice were cultured for 20 hr in the presence or absence of a water-soluble glucocorticoid, hydrocortisone sodium succinate. By two independent assays of cell viability or function, virtually all thymocytes from untreated hosts were inactivated by hydrocortisone sodium succinate, whereas most, but not all, thymocytes from hydrocortisone-treated hosts were inactivated. Thus, the thymocytes which survive after treatment of the animal with glucocorticoids are only partially resistant to the effects of glucocorticoids in vitro.