Interferon in the blood was rapidly cleared from the circulation after intravenous injection. Intramuscular injection of alpha-interferon caused low but stable interferon levels in the blood. However, in the case of beta-interferon, interferon was never detected consistently in the blood after intramuscular or subcutaneous administration. Intraarterial administration of beta-interferon also caused low but stable interferon levels in the blood. Our studies suggest that beta-interferon should be given intravenously to see clinical beneficial. No difference in pharmacokinetics was seen between natural interferon and recombinant interferon. No difference was also noted between partially purified interferon and highly purified interferon.