Rat thymic epithelial cells were cultured for 39 days in the presence of various concentrations of oestradiol, testosterone, progesterone and corticosterone and the supernatants assessed for effects on the stimulation of cells from the thymus, bone marrow, lymph nodes and spleen, with several agents. All the steroids, except progesterone, were found to significantly regulate the secretion of immunoregulatory factors by the epithelial cells at physiological levels but the effects were dose dependent. Fractionation of active supernatants indicated that the capacity to enhance or depress cellular proliferation was mainly associated with substances having molecular weights greater than 30,000 or less than 1000, respectively. This study supports the idea that certain steroids can influence the immune response indirectly through the thymus.