Abstract Physiological concentrations (10 −10 m) of l-thyroxine and triiodo- l-thyronine were found in vitro to enhance Ca 2+-ATPase activity in reticulocyte-enriched red cell membranes from female rabbits and to inhibit this enzyme in the male reticuloycte. Cross-incubation experiments with reticulocyte-enriched red cells and plasma from the opposite sex demonstrated that this sex-specific membrane response to thyroid hormone was transferable by plasma. Similar experiments with intact reticulocytes exposed to physiological concentrations (10 −11 m) of testosterone and 17β-estradiol indicated that the plasma factors were the sex steroids. That is, incubation in vitro with testosterone converted female-source reticulocytes to male-type responsiveness to thyroid hormone (inhibition of Ca 2+-ATPase activity); incubation with estradiol converted male-source reticulocyte-enriched red cells to female-type responsiveness (stimulation by iodothyronines of membrane Ca 2+-ATPase activity). Similar results were obtained when reticulocyte ghosts were incubated with testosterone and 17β-estradiol prior to determination of membrane enzyme activity. Etiocholanolone (5β-androstan-3α-ol-17-one) and testosterone were equipotent, but 5α-dihydrotestosterone had little activity in this system. Estrone and estradiol were equipotent, but estriol had no permissive effect on the stimulation by iodothyronine of reticulocyte membrane Ca 2+-ATPase activity. Expression of thyroid hormone action in vitro on Ca 2+-ATPase activity in the rabbit reticulocyte is determined at the membrane level by testosterone and estrogen. The structure-activity relationships of the sex steroids for this membrane action are different than those reported for nuclear actions of the steroids.