Human interferon (IFN) prepared from virus-induced human leukocyte suspensions (leukocyte-derived interferon) was compared to the IFN extracted from Escherichia coli harboring a human interferon-alpha cDNA hybrid plasmid (Hif-SN35-AH-L6). E coli-derived IFN was 20 to 50 times more active than leukocyte-derived IFN on heterologous bovine, feline, murine and guinea pig cells, relative to the activity on human cells. After partial purification by affinity chromatography on an anti-human lymphoblastoid IFN antibody column, the IFN was analyzed by SDS-polyacrylamide gel electrophoresis. While leukocyte-derived IFN gave a heterogeneous pattern with major peaks of activity of 24000 and 19000 daltons, E. coli-derived IFN gave a heterogeneous peak of activity at about 17-18000 daltons. The leading edge of leukocyte-derived IFN in SDS-polyacrylamide gels was significantly more active on bovine cells than on human cells and coincided in mobility with E. coli-derived IFN, which was also much more active on bone than on human cells. After reduction with mercaptoethanol in SDS, the E. coli-derived IFN lost no activity, whereas the leukocyte-derived IFN lost about 90% of its activity. After reduction, E. coli-derived IFN migrated in SDS-polyacrylamide gels as a single peak at 24000 daltons, as did the residual activity of reduced leukocyte-derived interferon. Out data suggest that the interferon produced by the E. coli harboring the clone Hif-SN35-AH-L6 is analogous in size and cross-species activity to one of the molecular species of leukocyte-derived interferon.