RT-4 tumor cells, derived from human carcinoma of urinary bladder, were destroyed following exposure to partially purified human fibroblast interferon (IFN). The cytocidal effect of IFN was detected after addition of more than 200 IU IFN/ml and incubation of cell cultures for 2 days or longer. This effect was observed as morphologic changes and decrease in either dye uptake as morphologic changes and decrease in either dye uptake or colony formation by the IFN-treated cells. The cytocidal response of RT-4 tumor cells to IFN was the most pronounced, whereas the responses of three other tumor cell lines [HT-29 (colon adenocarcinoma) and SAOS-2 and 5959 (osteosarcoma)] were markedly weaker. Diploid fibroblasts were completely resistant to the cell-killing effect of IFN. Susceptibilities of the four tumor cell lines and two normal fibroblasts strains tested to the three effects of IFN (cytocidal, antiproliferative, and antiviral) appeared to be distinct.