CAMA-1 cells, isolated from malignant pleural effusion, are grown in long-term cultures as monolayers. The rate of growth is dependent upon fetal bovine serum and estradiol. These cells also exhibit a dose response to added 17 beta-estradiol with respect to the incorporation of radiolabeled thymidine into cells. The uptake is increased at low levels of estradiol and decreased at pharmacological levels of estradiol. The uptake of uridine and leucine is also stimulated by estradiol in a dose-related manner. Induction of precursor uptake is not observable until cells have been exposed to estradiol for approximately 10 hr or longer. Cells plated for different periods in steroid-stripped serum remain sensitive to estrogenic stimulation and show similar lag time in response. Estrogenic effect is not noticeable in the absence of serum. Addition of prolactin can partially restore estrogenic stimulation of thymidine uptake in serum-free medium. Like other estrogen target tissues, these cells contain cytoplasmic and nuclear estrogen receptors. These results demonstrate that the CAMA-1 cell line is estrogen dependent and that these cells may provide a promising model for the in vitro investigation of the mode of estrogen action in human breast cancer.