We have investigated the effects of a combination in vivo treatment with thymosin alpha 1 (TA1) and murine alpha/beta interferon (IFN) on natural killer (NK) activity and on tumor growth in B-16 melanoma tumor-bearing mice. The results indicated that treatment with a single injection of IFN (3 x 10(4] 24 h before testing, enhanced NK activity in tumor-bearing mice if the test was performed 10 days after tumor inoculation, when the animals have normal NK responsiveness. On the other hand, the same treatment led to lower or no improvement of NK responses when the assay was performed 14 or 18 days after tumor inoculation, at a time when tumor growth caused NK-suppression. However, combination treatment with TA1 (200 micrograms/kg) for 4 days, followed by IFN was found to restore normal NK cell activity. On the other hand primary tumor growth was unaffected by combination therapy, while the same treatment with TA1 and IFN was able to significantly prolong survival time of B-16 tumor-bearing mice, when administered starting on day 6 after tumor inoculation. The last evidence, together with results on NK activity stimulation, indicates that combination therapy with TA1 and IFN could be an interesting approach to cancer immunotherapy.