The effects of human leukocyte interferon (Le-IF) and fibroblast interferon (F-IF) on the growth of the transformed human cell lines, RSa and RSb, were compared. Both were considerably more sensitive to F-IF than to Le-IF. Two cell lines, IF(r) and F-IF(r), derived from the RSa cell line, were resistant to the anticellular effects of Le-IF, but less resistant to those of F-IF.