Two transplantable granulosa cell tumor lines (designated as T and M lines) induced in mice by intrasplenic ovarian grafting were established. The hormonal milieu which promotes the growth of transplantable granulosa cell tumors was investigated in the present work. The inoculation of the T line tumors into host animals led to tumor formation in intact and gonadectomized animals, but not in hypophysectomized animals.Subcutaneous grafts in intact male hosts were significantly large than those in gonadectomized male hosts, whereas tumors of approximately equal size were formed in androgen-treated gonadectomized male hosts. These results indicate that both androgens and gonadotropins promote the tumor growth in vivo. The behavior of the M line tumors was similar to that of the T line tumors. When the cell growth of the M line tumors was examined in culture, steroid hormone depletion in medium did not affect the growth of the tumor cells, nor did androgen have a growth-promoting effect in serum-free medium or in medium supplemented with charcoal-extracted male mouse serum. It was concluded that the growth-promoting effect of androgens is not direct, and that androgens themselves or their derivative estrogens may produce specific growth factors, which in turn promote the tumor growth in vivo.