Oestradiol and progesterone receptors in the mammary tissue and tumours of female Holtzman rats following 7,12-dimethylbenz(a)anthracene administration.

A single dose of the carcinogen 7,12-dimethylbenz(a)-anthracene (DMBA) when administered to 55-day-old female Holtzman rats did not produce any change in the pattern or amount of oestradiol receptors in the breast tissues, when compared to age-matched control animals. The period of observation extended up to 280 days of age. Following DMBA administration to 102 animals, only 60% developed carcinomas of the breast, of which 59 and 50% were ER- an PgR-positive, respectively. This lower yield of steroid receptor-positive tumours in this strain of rats was also reflected in a very low incidence of regression of tumours to ovariectomy. Plausible explanations for both these findings have been offered.