All primary cell cultures tested from numerous human and rhesus monkey tissues were susceptible to Coxsackie A9 virus infection and were found to contain a specific A9 virus receptor. Ability of these cells to adsorb actively virus correlated with the presence of this receptor. Attachment of A9 virus to cell receptor was specifically enhanced by calcium and magnesium ions. Human cells established in continuous culture from pretested susceptible cultures were found to have lost A9 virus susceptibility and A9 receptors but not receptors for adsorption of type 1 poliovirus or B1, B3, B5 Coxsackie viruses. These cultures were able also to produce infectious A9 virus after exposure to viral RNA. Coxsackie A9 virus as well as type 1 poliovirus, inactivated following reaction with debris prepared from susceptible cells, could be recovered fully in infectious form by treatment of the inactive systems at low pH. Recovery of infectious A9 virus was also possible with a chelating compound. Coxsackie A9 virus could also be dissociated by means of low pH after adsorption onto susceptible cells for 2 minutes at 1°C., but not after 1 hour incubation at 37°C., indicating that viral host cell activity may be required in order for irreversible enterovirus attachment to take place.

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