A glucocorticoid sensitive T-lymphocyte cell line (CEM) was used to study in vitro interaction effects of steroidal compounds on their metabolic activity. Mixtures of steroidal hormones and related substances were added to the cells and assessed in short-term culture experiments by the incorporation of 3H dTr. Dose ranges were selected to include the linear portion of the dose-response curve of each of the potentially interactive substances. Evidence for synergy, additivism or antagonism was obtained for each of the various steroid combinations studied using algebraic and geometric calculations.