Regulation of cell division by cortycosteroid hormones will be investigated. Human and mouse cell lines sensitive to growth inhibition by steroid hormones will be used for these experiments. The timing of growth inhibition will be determined by means of pulses with labeled thymidine. At times corresponding to inhibition of DNa synthesis cell extracts will be prepared and assayed for inhibition of protein synthesis and for the appearance of two enzymes induced by interferon that may be involved in the catabolism of RNA and in the inhibition of protein synthesis respectively. Control experiments to study the specificity of the steroid hormoneswill be carried out by using an antagonist that binds to a steroid receptor. These experiments may reveal important features of the regulation of cell growth by hormones. In particular, if the induction of new enzymatic activities precedes the inhibition of cell growth, these findings will provide an important contribution of our understanding of the processes that regulate the proliferative activities of the cells. Further experiments will test whether the appearance of new enzymatic activities requires synthesis of protein or RNA, or is due to the release of these activities from preexisting inactive or sequestered forms of enzymes, as for example from lysosomes.