Human leukocyte interferon (IF) was found active in homologous human and heterologous rodent cells both in antiviral tests and in tests measuring cell-growth inhibitory activity. About 10 times more units of human IF were required for a 50% inhibition of multiplication of the sensitive LB2 subline of mouse L cells than for a comparable inhibition of these cells by homologous mouse IF. The BHK-21 line of hamster kidney cells and the CSV-21 subline of mouse L cells proved insensitive toward the cell-growth inhibitory action of human interferon, although both lines were sensitive toward it’s antiviral effect. An analysis of fractions obtained in PAGE of human IF indicates two types of profiles in human cells. One of them corresponds to the profile of IF observed in mouse or hamster cells. The results indicate that the antiviral and the cell-growth inhibitory effects of interferon, in both homologous and heterologous cells, are greatly influenced by the species of animal cells used for tests.