Trypan red, when injected intraperitoneally into mice, has been found greatly to lower the incidence of the infection of mice inoculated intraperitoneally with the neurotropic MM virus. The protective action of the dye is overcome if the virus is inoculated in too high concentration. The lowered incidence of infection was observed in mice inoculated with virus for as long as 29 days after the last dye injection. Of a number of dyes tested, trypan red, brilliant vital red, and Congo red were found effective.

In cotton rats inoculated intraperitoneally with MM virus, trypan red was likewise found to lower the incidence of infection.

With monkeys and a typical poliomyelitis virus no protection was observed against the virus inoculated intraperitoneally. The latter experiment is considered to have been inadequate for a critical test of the effect of trypan red on poliomyelitis infection.

When either the MM virus or Lansing virus were inoculated intracerebrally into mice, the effect of the dye on incidence of infection was small. In the case of the Lansing virus the difference was statistically significant, however. The possible relation of alteration in the permeability of the barrier between the blood and the central nervous system as a cause of the effect of trypan red is discussed.

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