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The results are reported of experimental investigations and epidemiological observations on the use of human leucocyte-produced interferon for influenza prophylaxis. Field trials with interferon were carried out during the outbreak of Hong Kong influenza in the USSR in January and February 1969. These covered about 14 000 people in comparable interferon-and placebo-treated groups and achieved an effectiveness of between 56.3% and 69.2% according to the age-group studied.The importance is shown, not only of artifically introduced interferon, but also of the interferon naturally produced by a subject in response to infection. The degree of such production varies both with each individual's capacity to form interferon and with the capacity of a particular strain of Hong Kong virus to elicit the productive response. The author suggests that isolated virus strains be subdivided into interferon-positive (I+) and interferon-negative (I-) strains according to the presence or absence of this phenotypic character.