There is well documented evidence both in humans and in experimental animals that exposure to diethylstilbestrol (DES) during pregnancy results in an increased incidence of tumours in the progeny. The increased cancer risk has been reported to persist in the second generation descendants of DES-exposed pregnant mice. In the present experiment, female mice of the CBA strain were treated at day 17 of pregnancy with 1 microgram/g body weight of DES. The descendants of DES-treated mothers, described as F1DES, were mated among each other or with untreated animals. The F1DES females were found to be sterile when mated with either F1DES or untreated males. F1DES males were successfully mated with untreated females. In the female offspring so obtained, but not in the male, a statistically significant increased incidence of tumours was observed, in particular of uterine sarcomas, and also of benign ovarian tumours and of lymphomas.