The antitumor effect of recombinant human interferon-beta (r IFN-beta) and recombinant interferon-gamma (r IFN-gamma) was studied in vivo using a pulmonary metastatic model involving nude mouse human colon cancer xenografts. The results indicated that both r IFN-beta and r IFN-gamma had an inhibitory effect on pulmonary metastases. Furthermore, a combination of r IFN-beta and r IFN-gamma acted synergistically in the inhibition of pulmonary metastases. These results suggested that a combination of r IFN-beta and -gamma could be a most effective form of interferon therapy for cancer.