Bilateral adrenalectomy and administration of thymosin (fraction 3) after adrenalectomy inhibit to different degrees the development of mammary gland tumors induced in rats by 7,12-dimethylbenzanthracene. The maximal effect was observed from the use of thymosin in conjunction with hypocorticalism; there was a substantial reduction in the frequency of developing neoplasms, the latent period of their development was lengthened, and their growth retarded. © 1980 Plenum Publishing Corporation.