Pretreatment of human fibroblast cultures with human leukocyte interferon (HIF, 1,000 IU/ml) resulted in a 24-h delay of virus replication after infection with vaccinia virus and herpes simplex virus type 1 and type 2. Additional HIF treatment 24 h after infection effectively lowered the maximum yield of viral infectivity. Equal results were obtained in simian cells with 3,000 IU of HIF per ml. The spread of two cell-bound herpesviruses, varicella zoster virus and Medical Lake macaque herpesvirus, was inhibited by 2,000 IU of HIF per ml in human fibroblasts and Vero cells, respectively. Varicella zoster virus infectivity was notably reduced by HIF, whereas the latter system showed a low sensitivity. To study the effect of HIF in the infected cornea, keratitis was induced experimentally in both eyes of 12 rhesus monkeys and 12 African green monkeys by inoculation with vaccinia virus and herpes simplex virus, respectively. In each monkey one eye served as a control for the full cycle of disease. In the other eye HIF treatment was initiated prophylactically 15 h before or simultaneously with the challenge virus infection or 6 to 20 h postinfectionally or therapeutically after onset of the disease, and the treatment was continued for 2 to 7 days. Prophylactic and simultaneous administration equally resulted in inhibition of both vaccinia and herpes keratitis. Postinfectional and therapeutic administration of interferon moderated the course of keratitis slightly and shortened the period of virus shedding.