This study examined whether the soluble tumor-associated antigens (sTAA) of 66 kDa and 51 kDa could promote suppression of chemically-induced rat mammary tumorigenesis by the hormone-related anticancer drug tamoxifen and prevent the drug's toxic side-effects. Dimethylbenzanthracene (DMBA, 10 mg/rat, 3 administrations) was used to induce mammary tumors in 8-week-old Wistar rats. Then, for 13-17 more weeks, preparations of sTAA (50 microg/rat) and tamoxifen (10 mg/rat) were administered, separately or in combination, on a weekly basis. The experiment was continued for 18 weeks and was terminated when the number of dead rats reached 50% in each group. Treatment with tamoxifen inhibited tumor growth and their malignance: the number of rats without malignant tumors significantly increased compared to controls, 27.3% and 5.6%, respectively. Treatment with sTAA resulted in a significant increase in the number of regressed tumors to 10.1% compared to 0% and 1.4% in control and tamoxifen-treated rats, respectively. Moreover, the period of 50% survival increased from 13 weeks in tamoxifen-treated rats to 17 weeks, and as a result, rats treated with sTAA were involved in the experiment for an average 14.3 weeks compared to 10 and 10.4 weeks in control and tamoxifen-treated groups. In rats treated simultaneously with tamoxifen and sTAA, the time of appearance of each new tumor increased from 4.5 weeks to 6.6 weeks with a significant increase to 14.3% in the number of regressed tumors. The period to 50% survival increased to 18 weeks, and these rats were involved in the experiment for up to 16.4 weeks. The number of rats without malignant tumors increased to 22.2% and the time of appearance of malignancy increased to 9.6 weeks, as compared to 7.3 weeks in controls. The results demonstrated that sTAA have tumor-suppressive properties, and also enhance the anticancer effects of tamoxifen and prevent its toxic side-effects.