The aim of the present investigation was to study the influence of sex of tumour bearing mice and of ovariectomy on the proliferation kinetics of a solid, transplantable mammary carcinoma (C3H-mouse). According to the growth curves the tumour growth was delayed in spayed female and in male mice compared with normal female animals. This was due to a longer cell cycle time of tumour cells in the former, mainly caused by a prolonged presynthetic gap (TG1). Concerning the growth fraction and the cell loss factor, essential differences could not be detected. Thus, the solid tumour displayed a distinct though partial disengagement of growth from hormonal dependence.