was a possibility that the abnormally high parasitemia observed in Swiss mice was a reflection of a Swiss-adapted parasite. The hypothesis was tested (Greenberg and Kendrick, 1957a) by passing the parasite serially through each of the strains of mice. There was no evidence that in any of these strains of mice there was an increase in parasitemia with continued serial passage. The high parasitemia in Swiss mice was then not entirely a matter of parasite-adaptation. It was thought that the degree of infection of mature erythrocytes achieved in each strain was genetically controlled. Something could be learned of the mechanism of inheritance of resistance to parasitemia by studying hybrids and backcrosses of the 2 extreme examples, Swiss and STR. It was expected that segregation and recombination of genes could most easily be determined with strains showing the largest differences.