Human tumor-degenerating factor (TDF), which is produced in the culture of human fibroblasts, causes the degenerative changes of the tumor cells and forms a cell-free area on the surface of culture dish. Its degenerating activity is enhanced by the treatment with interferons. In the present study, it was recognized that TDF preparation inhibited the colony formation of KB cells according to the TDF activity. When recombinant human leukocyte interferon (rHuIFN-alpha A) was added to the KB cell culture, the colony formation was inhibited. The combination of TDF preparation with rHuIFN-alpha A inhibited more strongly the colony formation than TDF alone or rHuIFN-alpha A alone. It was found that the combination of TDF preparation with rHuIFN-gamma inhibited more remarkably the colony formation than the combination of TDF preparation with rHuIFN-alpha A.