1-Adamantanamine hydrochloride (aminoadamantane) has been shown to inhibit the multiplication of influenza A viruses in cell cultures, organ cultures and experimentally infected mice. An epidemic of influenza in Great Britain in the winter of 1967-68 provided the opportunity to study the prophylactic effect of aminoadamantane in the family environment and this paper reports the results of further observations in January-March 1969 during an influenza epidemic due to A2/Hong Kong/68.In a double-blind, placebo-controlled investigation of aminoadamantane given prophylactically, in doses of 100 mg every 12 hours for 10 days, no protective effect of the drug was demonstrated. In view of the positive effect of the drug in the earlier study during the winter of 1967-68, and the apparent sensitivity of the virus strain, an explanation involving the very high incidence of individuals possessing HI antibody of less than 1:12 is offered.