Publisher Summary Human interferons are a heterogenous group of proteins that are classified broadly into Hu-IFNα (leukocyte, lymphoblastoid), Hu-IFN−β (fibroblast) and Hu-IFN−ψ (immune interferon). A large part of the clinical work has been conducted with Hu-IFN−α, although Hu-IFN−β has also been used. Hu-IFN−α can be obtained from primary leukocytes, from lymphoblastoid cell lines, and from genetically engineered bacteria. Because of the potential limit on the quantity of donor leukocytes, this does not seem likely to be a suitable source for large-scale manufacture. Interferon has usually been given to man as an i.m. injection. Doses in adults of 5 million units (1 x 105/kg) achieved mean blood levels between 1 and 12hours of 50 units/ml serum. A comparison of different routes shows that following an i.v. infusion of 30 million units over 5 minutes, the half-life was about 15 minutes during the first hour and about 90 min between the first and fourth hours. The interferon was essentially cleared within 6 hours. The same dose given as an infusion over 8 hours produced a rising blood level during the 8 hours to reach a peak between l0 2 and l0 3 units.ml, but the blood level had declined to insignificant levels by 24 hours. Following an i.m. injection of 1 million units in some 20 subjects, the peak was seen at 2 hours and the level remained around 100 units for 6 hours. The pattern following S.C. injection was very similar to the i.m. pharmacokinetic profile. In a careful study carried out with lymphoblastoid interferon, it has been demonstrated that doses of 5 mega units/m 2 maintain an average blood level of 150 units over 24 hours. No interferon has been detected in the cerebrospinal fluid (CSF).