Steroid hormone regulation of proliferation of the Ishikawa human endometrial adenocarcinoma cell line was investigated in defined tissue culture medium. Oestradiol increased cell number following treatment for greater than 8 days; 4-OH tamoxifen, used alone, induced growth in a similar manner to oestradiol and was not antagonistic when used in combination with oestradiol. Progesterone decreased cell number 4 days after treatment but thereafter the effect was lost; the effect of progesterone was abolished in the presence of Phenol Red, consistent with the oestrogenic properties of this indicator. Oestradiol together with progesterone for greater than 8 days resulted in maximal growth and was preceded by an apparent increase in synthesis of a protein of molecular weight 36 kDa pI 8.