The observations of Stokes, Bauer and Hudson1 that Macacus rhesus can be easily inoculated with the virus of yellow fever in Africa and that this disease is not due to Leptospira icteroides Noguchi, have stimulated renewed interest in research on yellow fever in America, where most of the classic studies on this disease have been made. The occurrence in June, 1928, at Rio de Janeiro of several cases of yellow fever presented the opportunity for the experiments reported here. Only four Macacus rhesus monkeys were available in Rio de Janeiro when the first cases of yellow fever appeared. One of these (monkey 1) was inoculated with 2 cc. of a culture of Leptospira icteroides, Palmeiras strain, isolated by Noguchi and his co-workers;2 the other three (monkeys 3, 4 and 5) were inoculated with blood drawn from patients with yellow fever two and one-half, three, and three and one-half.