The effects of estrogen, antiestrogen and ovariectomy on the growth of a mouse teratocarcinoma, OTT6050, were studied in vivo. Subcutaneous tumors transplanted in syngeneic 129/Sv male and female mice were measured, and tumor-growth curves were constructed using the products of three principal tumor diameters, designated tumor volume. There was a linear correlation between the measured tumor volume and the actual excised tumor volume. The tumor growth rate in estrogen-treated groups was significantly greater than that in the control group, and the difference in tumor volume between treated groups and controls increased with time. However, dose responsiveness was not observed. In antiestrogen-treated or ovariectomized mice, tumor growth was partially inhibited only at the early stages. The mean survival time of estrogen-treated and ovariectomized groups was shorter than that of controls, while that of the antiestrogen-treated group was identical to that of controls. Histologically, no specifically different types of differentiated tissues were observed in any group. Our experiments show that estrogen is one important factor which stimulates the growth of one form of teratocarcinoma.