Interferons (IFNs) may modulate oestrogen (ER), progesterone (PR) and epidermal growth factor (EGFR) receptor expression in vitro. ER, PR and EGFR levels in tumour specimens taken from 13 patients with endometrial adenocarcinomas before and after 5 days' intramuscular treatment with 5 x 10(6) U per recombinant human leucocyte interferon-alpha 2b (rh IFN-alpha 2b). After treatment, ER (P less than 0.01) and PR (P less than 0.05) levels were significantly increased with a simultaneous reduction of EGFR content (P less than 0.05). Since the expression of ER and PR characterises more differentiated hormono-sensitive tumours, while EGFR are preferentially expressed in less differentiated tumours, the increase of steroid hormone receptor levels with the reduction of EGFR expression suggests that rh IFN-alpha 2b may induce endometrial cancer cell differentiation. Moreover, the decrease of EGFR levels may explain the antiproliferative effect of IFNs.