More than half of the monkeys maintained for various periods on a modified dietary regimen deficient in vitamin B became more or less anemic. All developed a granulopenic leucopenia during which many developed spontaneous infections, and all were extremely susceptible on intranasal instillation of Streptococcus hemolyticus, group C.

In normal monkeys, primary inoculation with the streptococcus resulted in an immediate marked neutrophilic leucocytosis. In the nutritionally deficient animals with leucopenia, only an abortive transitory leucocytosis was observed.

Following reinoculation with the same strain of hemolytic streptococci in normal healthy monkeys, the prompt leucocytic response originally observed, failed to develop, yet the animals remained asymptomatic. In the first instance the opsonic index remained low at the preinoculation base line level, but in the second, a sharp increase in the opsonic index occurred in several days. This humoral response apparently rendered unnecessary the quantitative granulocytic leucocytosis associated with survival in the first instance. There was no demonstrable difference in time of appearance or titer of the antibody responses, i.e. precipitins, antistreptolysins, and opsonins between the normal and nutritionally deficient monkeys.

Synthetic folic acid (L. casei factor) relieved both leucopenia and anemia promptly in the "vitamin M"-deficient monkeys, with increase in resistance to endogenous infection when this supplement was given sufficiently promptly.