The effect of chronic exposure to diethylstilboestrol (DES) over the concentration range 10(-4)-10(-6) M on the in vitro growth of cells derived from the squamo -columnar junction of the human cervix has been examined. DES over the dose range 10(-4)-2 X 10(-5) M inhibits cervical epithelial cell (HCE) growth in terms of colony formation and colony expansion. This effect involves both an effect on cell proliferation, as measured by cell number and mitotic index, and an induction of differentiation as measured by an increase in the number of PAS positive cells (glycogen accumulating cells) and in the number of cells with cornified envelopes. At lower doses of DES, 10(-5)-5 X 10(-6) M, the efficiency of colony formation and colony size are not inhibited but mitotic index and cell number are decreased and the fraction of differentiating cells is increased. The effects of DES on HCE cell growth can be modified to some extent by epidermal growth factor (EGF). EGF at 10 ng/ml, increases the fraction of proliferating cells, induces cellular hypertrophy, reduces stratification and the fraction of differentiating cells in HCE colonies. DES, at all concentrations other than 10(-6) M, in the presence of EGF inhibits cell proliferation but does not prevent the EGF induced cellular hypertrophy, reduction in stratification or reduction in the fraction of differentiating cells.