Summary L strain fibroblasts were grown in medium 199P containing estrone-16-C14 and estradiol-16-C14 at a concentration ranging from 10-7 to 10-5 M. These cells did not convert either steroid into water soluble conjugates in 72 hours. The estradiol in the medium and that associated with the cells was unchanged, but a large portion of the estrone was converted to estradiol. In experiments using cell suspensions, some of the early events in the interaction between estradiol and fibroblasts were studied. The primary reaction appeared to involve a rapid adsorption of the compound to the cell surface. There is evidence indicating that very little, if any, of the steroid penetrated the cell. The amount of estrogen that adsorbs to the cell surface could be directly related to the estradiol content in the medium. When estradiol content in the medium was 3.5 × 10-5 M, there was inhibition of cell growth. This inhibition has been previously shown to be associated with the blockage of substrate transport. Approximately 109 molecules per cell were required to effect complete inhibition of cell growth. Adsorption of estradiol could be impaired by addition of horse serum to the medium, and under such conditions a greater amount of estradiol was required to attain the extent of inhibition observed in a serum free medium. The competition by serum proteins occurred between cell surface sites and proteins in the medium for free steroid rather than between serum protein and steroid for surface sites.