There are numerous attempts to find novel anticancer drugs or to improve therapeutic protocols based on application of chemotherapeutic agents and immunomodulators (biological response modifiers, cytokines, various plant or bacterial products). Among the preparations that have beneficial effects for the cancer bearing organism are preparations of spleen peptides (Polyerga). Hence, we analyzed if treatment with spleen oligopeptides GP-1 (active substance for the manufacture of Polyerga ampoules' solution injected as 0.5 microgram/kg every second day) if given alone or combined with chemotherapy (Endoxan 50 mg/kg single i.p. dose) of mice bearing artificial lung metastases of mammary carcinoma will have an impact on the metastases count and survival of the animals. The results obtained have shown that chemotherapy reduced metastases count and increased survival of the tumor bearing mice, while the use of GP-1 alone did not affect metastases development. However, combined GP-1 treatment and chemotherapy were more efficient in prevention of the metastases development than chemotherapy alone. Thus, in mice treated with GP-1 and Endoxan, the average metastases count was four times lower than in the mice treated by chemotherapy only, while 2/12 animals were without tumor nodules in the lungs. Finally, all the animals treated by chemotherapy alone died until the 42nd day after tumor transplantation, while at the same time, only 5/10 animals died receiving combined therapy. Thus, these results give an experimental support for the use of the spleen peptides in biotherapy (or combined therapy) of cancer.