1.Experimental Trypanosoma brucei infection in Peromyscus leucopus noveboracensis (American forest deer mouse) runs a chronic course. Several P. l. nove-boracensis, in spite of being reinoculated two to five times with massive doses of virulent Tr. brucei, lived as long as 617 days. In a few cases, a feeble infection was detectable during this period.
2.Massive reinoculations, even at frequent intervals, failed, with a few exceptions, to provoke a definite relapse of the trypanosomes. The number of parasites in the circulating blood was small, and it was necessary to resort to inoculations of susceptible animals in order to prove their presence.
3.The forest deer mouse exhibits a marked degree of resistance to Tr. brucei. This host is an unfavorable environment for the parasite and Tr. brucei is gradually but markedly attenuated, which was manifested by the prolonged incubation periods in the test animals.
4.When Tr. brucei is kept in unfavorable media (in vitro or in vivo), susceptible animal inoculation tests require careful observations over a longer period than is usually considered sufficient.
5.After the last reinoculation of the forest deer mice, the Tr. brucei were gradually attenuated, and the incubation period in susceptible animals was again prolonged. At times the blood of P. l. noveboracensis failed altogether to produce infection in susceptible test animals (Rattus norvegicus and Mus musculus). The noveboracensis regained a high degree of relative immunity; their blood had acquired a pronounced degree of multiplication-inhibiting and trypanolytic properties.
6.P. l. noveboracensis, because of its inexpensiveness and because of the small amount of space needed when experiments are conducted on a large scale, is admirably suited for cross-immunity tests and for studying the ultimate fate of pathogenic trypanosomes in a reservoir host.