1. It has been possible to introduce yellow fever virus into the small Brazilian monkeys, Callithrix albicollis and Leontocebus ursulus, by the bites of infected mosquitoes and to carry the virus through a series of four passages in each species and back to rhesus monkeys by the bites of Stegomyia mosquitoes fed on the last marmoset of each series.

2. Five specimens of L. ursulus were used. Four developed fever, and all died during the experiments. At least two showed liver necroses comparable to those found in human beings and rhesus monkeys that died of yellow fever.

3. Twenty specimens of C. albicollis were used. Very few showed a temperature reaction following the introduction of virus. Of those that died, none had lesions typical of yellow fever as seen in certain other species of monkeys and in humans.

4. The convalescent serum from each of five C. albicollis protected a rhesus monkey against yellow fever virus, but the serum from a normal marmoset of the same species was found to be non-protective.

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