Murine L1210 leukemia cells were cultured in the presence of various steroid hormones to determine their growth response. Cells maintained in medium containing glucocorticoids such as dexamethasone exhibit a 10--20% inhibition of growth, whereas progesterone of 17 beta estradiol produce a 40--50% inhibition of growth. The response is due to growth inhibition and not to any cytolytic effect of the steroids. L1210 cytoplasmic and nuclear extracts contain high affinity binding sites for [3H]dexamethasone, [3H]progesterone and [3H]17 beta-estradiol. Competition with various steroids indicate that estrogens bind to one class of cytoplasmic binding sites, while glucocorticoids and progestins bind to a second class of receptor sites. These receptors appear not be be completely responsible for growth inhibition since there is no correlation between the magnitude of growth inhibition and the intracellular concentration of receptor sites, and since the concentration of steroid required to cause significant growth inhibition exceeds those concentrations required to saturate the cytoplasmic receptors.