1. In infections with Trypanosoma lewisi in the rat there are three manifestations of resistance against the parasite: (1) the retardation and final inhibition of reproduction of the trypanosomes by about the 10th day; (2) a sudden drop in the number of parasites in the blood between the 9th and 15th days; and (3) the total disappearance of the parasites, which terminates the infection and occurs from a week to several months after the first drop in numbers.

2. The first manifestation of resistance, vis. the retardation and final inhibition of reproduction, is the result of the formation of a reaction product in infected rats which inhibits reproduction of the trypanosomes but does not kill them.

3. No product similar to, or identical with this reaction product can be demonstrated in in vitro cultures of Trypanosoma lewisi.

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