Interferon level in human normal sera

Interferon (IFN) level in the sera of normal humans was determined by their protective effect on a KB cell culture against the cytopathic effect (CPE) induced by vesicular stomatitis virus. The technique revealed all the three known types of IFN. In the investigated population 15 of 37 sera (40%) were found to protect against the CPE at 1/10 dilution, 11 of 37 (30%) still protect at 1/20 dilution, and 7 of 37 sera (19%) still protect at a dilution of 1/40. No serum was found to have detectable IFN levels at 1/80 dilution. We can thus consider that the normal serum level ranges up to a 1/20 dilution i.e., up to 20 IU/ml. The above titers are slightly higher than the normal IFN level reported by other investigators in other human populations, although no clear reasons can be given to account for the differences.