The international standard for anti-smallpox serum.

At the request of the WHO Expert Committee on Biological Standardization, the National Institute for Medical Research, London, obtained 63 samples of convalescent human plasma from patients recovering from smallpox. The sera were pooled and distributed into ampoules and freeze-dried.The freeze-dried material was examined in 11 laboratories in 6 countries in an international collaborative assay during 1964 and 1965.The results from 10 laboratories were more or less consistent, but relative potencies in terms of the proposed international standard estimated by one of the laboratories were exceptionally low. Since this laboratory used a strain of virus different from that used in all the other laboratories, it was suspected that antigenic properties or avidity of the virus might have caused the divergence of results; this is being investigated.After considering the report of the international collaborative assay, the WHO Expert Committee on Biological Standardization established the material as the International Standard for Anti-Smallpox Serum, and a unitage was assigned so that 1 International Unit of activity was contained in 0.08416 mg, and each ampoule contains (on average) 1000 IU of activity.