The MXT tumor is an experimental mammary tumor, maintained by transplantation in B6D2F1 mice, which contains significant amounts of estrogen and progesterone receptors. The aim of the present work was to study the effect of estradiol (E2) administration on the cell kinetic parameters (thymidine labeling index: TLI, S phase duration: TS, cell cycle duration: Tc, growth fraction: GF, potential doubling time: T, cell loss factor: luminal diameter) of an MXT anaplastic carcinoma strain. These parameters were analyzed using single-, double- or repetitive-tritiated thymidine (3H-TdR) labeling techniques. Uteri were used as controls for the methodology. Our data clearly demonstrate that E2 exerted a marked stimulatory effect on the tumor cell proliferation. This effect is the net result of a shortening of the mean cell cycle duration (Tc) and of a 3-fold increase of the number of cells produced per time unit in 89% of the total cancer cell population. Beside this stimulatory effect, E2 produces a marked increase of the tumoral cell loss. This loss seems to affect a cell fraction which has reached the quiescent state (Q) for a short time.