Interferon was measured at different intervals after the iv, sc, and im application of exogenous human leukocyte interferon to patients with various virus diseases or neoplasms. Interferon injected iv into patients had a half-life of about 15 minutes in the 1st hour and of about 90 minutes in the next 3 hours. Six hours after iv injection of 30 million U, no serum interferon was detectable. With a continuous iv infusion, a relatively high serum interferon level was reached. By the im administration of 1 million U interferon, a peak level of serum interferon (mean value 107 U/ml serum) occurred after 2 hours and was fairly stable for about 6 hours. Twenty-four hours after im application, a low level of serum interferon was still detectable. Similar results were found after sc interferon injections. In a patient with subacute sclerosing panencephalitis, no interferon was found in the cerebrospinal fluid at the time of the highest serum interferon level and 24 hours after two im interferon injections. Only minimal side reactions resulted from sc and im interferon injections. In one patient, a shock reaction occurred after iv application. For therapeutic trials, about 1 million U exogenous human interferon should be injected twice daily im or sc.