A further analysis of the growth curve obtained in vitro for influenza virus in chorioallantoic membrane has been made using the viral inhibitor p-fluorophenylalanine. It has been found that p-fluorophenylalanine is phase-specific, does not interfere with the initiation of infection but rather acts during the productive period of the infectious sequence. The site of action of this fluoroderivative has been described relative to the site of action of methoxinine. By the use of this inhibitor in combination with methoxinine, it has been possible to recognize two stages of development which were not discernible from the usual growth curve. In a multicellular culture these two reactions, A and B, occur for the most part simultaneously, during the latent and productive periods. Reaction A is inhibited by methoxinine, but not by fluorophenylalanine. It begins early in the latent period and can proceed independent of reaction B. Reaction B is inhibited by fluorophenylalanine, but not by methoxinine, and it cannot proceed unless reaction A is proceeding or has operated for some prior interval.

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