liparous or 8-month-old multiparous C3H/HeJ mice with 0.1 mg of 2-bromo-a-engocryptine (CB-154) on 6-methyl-8-f3- ergoline-acetonitmile, efficacious inhibitors of prolactin se cretion, markedly reduced the incidence of spontaneous mammary hyperplastic nodules and mammary tumors. CB

Daily treatment (for 12 to 14 months) of 2-month-old nul liparous or 8-month-old multiparous C3H/HeJ mice with 0.1 mg of 2-bromo-a-engocryptine (CB-154) on 6-methyl-8-f3ergoline-acetonitmile, efficacious inhibitors of prolactin se cretion, markedly reduced the incidence of spontaneous mammary hyperplastic nodules and mammary tumors. CB 154 appeared to be more effective than 6-methyl-8-/3-engo line-acetonitnile in suppressing the incidence of mammary tumors; the ergot virtually prevented the appearance of mammary tumors in nullipamous mice. Daily treatment of 5month-old estrogen-treated , ovaniectomized-hysterectom ized C3H/HeJ mice for 12 months with CB-154 also signifi cantly reduced the incidence of hyperplastic nodules and mammary tumors when compared with ovaniectomized-hys terectomized mice treated with the steroid alone. Daily treatment of multiparous C3H/HeJ mammary tumor-beaning mice with CB-154 or 6-methyl-8-f3-engoline-acetonitnile gen emallyfailed, however, to promote regression of the mam many tumors. Thus significant prophylaxis of early pneneo plastic lesions by drug-induced hormone (prolactin) suppression, resulting in a marked reduction in mammary tumor incidence, has been demonstrated in this study.