A fixed neurotropic strain of yellow fever virus, in its 250th mouse brain passage, was maintained in saimiri monkeys for 22 serial passages by the intramuscular inoculation of a 1:10 dilution of the 4th-day serum of the preceding animal. The response of the monkeys to infection was similar to that observed in passage experiments with pantropic strains. Eleven of the 22 apparently died as a result of virus infection; 6 of the 11 showed signs of stomach haemorrhage; in no instance did postmortem liver tissue show pathological lesions characteristic of yellow fever; in no instance was there a well-defined febrile reaction. The virus showed no demonstrable change in behavior after intracerebral inoculation in white mice in the course of the passage experiment: the mouse incubation period continued short and regular, characteristic of the “fixed” virus. There was no regular increase in titer of circulating virus in monkeys in successive passages; the strain seemed to lose virulence, since 8 of the first 9 infections were fatal in contrast with 3 of the following 13.
Eight attempts to infect haemagogus mosquitoes on these monkeys failed completely: virus was not recovered by inoculation of the mosquitoes into mice or into saimiris, or by feeding them on baby mice or saimiris except in one instance which was probably due to a confusion of mosquitoes infected with another strain of virus.