1. A considerable variety of dyes (azo, quinone-imide and xanthene dyes) have been tested for their effect upon influenza virus in vitro. Only certain dyes in the quinone-imide group seem to have an effect upon the virus, namely the oxazins brilliant cresyl blue, nile blue A and cresyl violet, and some containing the azin-safranine radical, namely janus purple, janus blue G, janus green B and diethyl safranine azo-4-(1-phenyl-3-methyl-pyrazalon-M-sulfonamide). Nile blue A, cresyl violet and janus green B are the most effective of the dyes.

  2. Only janus green B and the safranine-pyrazalon-sulfonamide have an effect in vivo in altering the course of infection with influenza virus in mice; this effect is evident only when the chemical is injected intravenously and in dosage close to the LD50 of the chemical. Janus green B has a more pronounced effect than does safranine-pyrazalon-sulfonamide.

  3. The effect of injection of these two chemicals is evident in the greater per cent of survival beyond ten days in the treated mice as compared to the control mice, and in the lesser per cent of the treated mice dying before the average period of survival of the control mice. In both of these comparisons the differences are statistically significant. In addition, the average period of survival is greater in the treated mice.

  4. These differences in effect upon the course of infection with the PR8 strain are noted when injection of the janus green B is started at three hours, 24 hours, and 48 hours after inoculation of the mice. With safranine-pyrazalon-sulfonmide the injections started at 48 hours are not effective.

  5. The two agents are more effective against the PR8 strain than against the WS strain of influenza virus.

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