Inhibitory effects of two human interferon preparations, leukocyte interferon (Le-IF) and fibroblast interferon (F-IF), on granulocytic progenitor cells (CFU-C) from hematologically normal cancer patients and from patients with chronic myelogenous leukemia were evaluated. There was a wide variation in sensitivity of CFU-C to both Le-IF and F-IF. F-IF was more inhibitory against CFU-C than Le-IF. Normal CFU-C and chronic myelogenous Leukemia CFU-C were equally inhibited by both interferons. Effects of both interferons were neutralized by corresponding specific antisera but not by the other antisera. These observations confirmed that differences in immunogenicity of the interferons may attend their different origins.