The rate has been determined at which the initial stable complex is formed between cells of the chorioallantoic membrane and the PR8 strain of type A influenza virus.
Characterization of the complex revealed the following properties: (a) stability to dilution and to simple washing with buffered saline, (b) insensitivity to the action of receptor-destroying enzyme (obtained from cholera vibrio), (c) sensitivity to the inhibitory effect of anti-viral immune serum, (d) formation of the complex at low temperatures, 3°C., (e) accompanied by a loss of detectable viral infectivity from the reaction mixture, (f) stability at 3°C. but transformation at 37°C. to a state which is resistant to the inhibitory action of a specific anti-viral serum. The rate of this latter transformation has been determined. The infectious process in more than 50 per cent of the cells can be influenced by immune serum 2 hours after the completion of the initial reaction with virus.