Receptors for oestrogen (ER) and progesterone (PR) were assayed in tissue from 17 patients with colorectal cancer and five colonic cancer cell lines using enzyme immunoassays. ERs and PRs were detected in 15 and 17 cancers respectively, although the levels detected were low: median (range) ER 1.3 (0–11.3) and PR 3.9 (0.3–10.2) fmol per mg protein. These values were not significantly different from median (range) levels of ER (1.1 (0.6–3.0) fmol/mg) and PR (19 (0.5–3.2) fmol/mg) detected in normal mucosa. There were significant positive correlations between the levels of ER and PR for cancer tissue (. = 0.56, P < 0.005; r(log transform) = 0.68, P < 0.003; n = 17) but not for mucosa, and between levels of ER in cancer tissue and mucosa (. = 0.55, P < 0.05; r(log transform) = 0.70, P < 0.025; n = 10) but not between the corresponding PR values. In maintenance media, which contained phenol red and unstripped fetal calf serum, the median (range) concentration of ER was 1.9 (1.2–10.4) fmol/mg and that for PR 24.3 (9.1–63.2) fmol/mg in the five cell lines studied (HT-29, LS174T, SW620, Lo Vo, COLO 320DM). The addition of oestradiol (10 nmol/l) to phenol red-free medium containing 5 per cent dextran-coated charcoal-treated fetal calf serum had little effect on the concentration of ERs or PRs in SW620, LoVo and COLO 320DM cells after 7 days' culture. It is concluded that ERs and PRs are expressed in malignant and normal colonic mucosa. ERs appear to be a feature of the colonic mucosa rather than the malignant process, but in carcinoma may regulate synthesis of PRs, suggesting a degree of oestrogen responsiveness.