The MXT transplantable mammary tumor is an ovarian sensitive carcinoma of the B6D2F1 mouse strain. The present work documents the mitogenic effect of a single near-physiological dose (pulse) of estradiol (E2) in castrated mice bearing MXT neoplasms 2-7 weeks after their inoculation. Histological sections of tumors and uteri (as control E2 target organs) were processed and nuclear thymidine labeling indices (TLI) were determined by autoradiography. Tumor estrogen receptors contents were evaluated in parallel. The results indicate that the MXT tumor keeps a constant sensitivity to E2 6 weeks after implantation.