From 9 to 10 of 10 cynomolgus monkeys infected with rabies street virus died of rabies about 20 days postinfection (pI). Symptoms of illness appeared 1 to 4 days before death. In an attempt to protect infected animals from the disease, human leukocyte interferon (HIF) was administered intramuscularly (i.m.) near the site of infection or into the cerebrospinal fluid between the first and second lumbar vertebrae (i.e., intralumbarly [i.l.]). Multiple HIF doses given over a period of several days proved more effective than a single HIF dose. In every experiment, i.m. HIF treatment was started 1 day pI. The best result obtained was a survival rate of 7 of 10 monkeys. The i.l. HIF administration schedules, consisting of multiple doses given over a period of at least 8 days, were started on day 3, 7, or 11 pI. Here the best result noted was the protection of 5 of 10 treated monkeys. The latest successful postexposure i.l. HIF treatment began on day 11 pI. The highest protection rate, 8 survivors of 10 treated monkeys, was achieved by a combined i.m. and i.l. HIF treatment. From these results we conclude that human patients severely bitten by rabid animals should in addition to an active immunization be i.m. and i.l. treated with HIF. Particularly, i.l. HIF administration could be effective, even when given several days pI. Whether an HIF administration starting after the appearance of clinical symptoms of rabies can help cannot be decided upon from the studies made in this monkey model. The most obvious difference between rabies in humans and cynomolgus monkeys is the duration of illness between the outbreak of the disease and death (1 to 4 days only in this animal model). It might have been due to this short period of illness that i.l. and i.m. HIF treatment at the appearance of clinical symptoms failed to help any of the monkeys treated.