SUMMARY: In tests of vaginal cornification, anti-oestrogenic activity has been demonstrated in dimethylstilboestrol, ethylstilboestrol and n-propylstilboestrol. It has not been detected in some other stilboestrols. Dimethylstilboestrol, the most potent anti-oestrogen, inhibits the increase in vaginal mitosis and epithelial thickness which normally follows the administration of oestrone to the ovariectomized mouse. This contrasts with the action of substances like testosterone and progesterone, which inhibit cornification but do not inhibit mitosis in ratios of up to a million to one.