We find that St. Louis encephalitis virus cultured in 10 per cent serum-Tyrode solution plus brain tissue from 1-day-old innately susceptible mice attains a higher titre than when cultured in a similar solution plus brain tissue from 1-day-old closely related, yet innately resistant mice. This difference in titre persists regardless of whether the serum comes from innately susceptible or resistant mice. The relatively high titre of virus in the susceptible media is not affected by the addition of an extract (not cell-free) from the resistant brain; the relatively low titre of the virus in the resistant media may possibly be slightly enhanced by the addition of an extract from the susceptible brain.

The findings as a whole show that the marked difference in the increase of St. Louis encephalitis virus in the brain tissue of innately susceptible and resistant mice, on culture in vitro, is due to some difference in the brain tissue itself.

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