Apparently as a result of repeated passages through guinea pigs, the virulence of surra of Mauritius for mice became greatly attenuated. Three mice recovered spontaneously and two remained carriers of tryanosomes for four to seven weeks after the parasites were last seen in their blood. The mice that recovered acquired no lasting immunity. While the virulence of the parasites decreased for mice, it increased for guinea pigs. By cultivating the parasites in mice the virulence for these animals was restored after four to eight passages.