The antiviral and antiproliferative activities of human interferon-omega (IFN-omega) on two human cell lines and on VERO (monkey), MDBK (calf), SPEV (pig), L929 (mouse), BHK-21 (hamster), and MDCK (dog) cell lines were compared with those of human IFN-alpha 1 and IFN-alpha 2. The results are tabulated. Compared with its antiviral titer on human A549 cells, INF-omega was more active on mouse cells and even more active on the pig cells, but had little activity on the hamster cells and virtually none on the dog cells. IFN-omega also inhibited the growth of all these cells to a greater or lesser extent, and there was in general an apparent correlation between its antiviral and antiproliferative activities on the different cells, except that the dog cells were relatively much more sensitive to the antiproliferative effect.