The biological activities of two thymic factors, serum thymic factor thymulin normally present in serum and thymosin alpha-1 (Ta-1) extracted from the thymus gland, have been studied. The effects of the factors on the growth of pulmonary metastases and survival of mice were evaluated in pathogen-free C3H/fSed males. Mice were injected i.v. with the single cell suspension of the syngeneic methylcholanthrene-induced fibrosarcoma. The treatment with thymulin and Ta-1 started two days after injection of 5 x 10(4) to 2 x 10(5) tumor cells per mouse. Different doses of the thymic factors were administered S.C. in sets of 5 daily injections through a period of 2 or 3 weeks. Numbers of tumor colonies in the lung were determined two weeks after the cell injection. Treatment with 0.1 micrograms Ta-1 per injection through the period of two or three weeks, prolonged the survival of tumor-injected mice. Similar effects were observed in mice treated with 0.01 microgram thymulin per injection. Numbers of tumor colonies in lungs of these mice two weeks after the cell injection were also reduced in comparison with saline-treated controls. These findings correlated with prolonged survival time of identically treated mice. The effectiveness of thymic factors in reducing tumor growth was dependent on the tumour load. In addition, the effects induced by Ta-1 persisted longer than observed in thymulin-treated mice. Mice challenged 150 days after the primary tumor cell injection and treatment with Ta-1 demonstrated increased resistance to tumor, while mice treated with other factors behaved as saline-treated controls. The results indicate that both factors exert beneficial effects against tumor growth, although mode of action for each factor may be different.