Matsumoto Eosinophilia Shinshu (MES) is a rat strain that spontaneously develops eosinophilia and eosinophil-related inflammatory lesions in many organs. In a previous study, we performed chromosomal mapping of the gene for eosinophilia in MES rats using backcross progeny and found that the major locus for eosinophilia was located on chromosome 19. In addition, another quantitative trait locus showing suggestive linkage for blood eosinophil count was found on chromosome 2. In this study, we examined additional marker loci in the backcross progeny and discovered that a third locus for eosinophilia was also located on chromosome 1. These data reinforce the notion that eosinophilia in MES rats is a rather complex genetic trait. However, these results will form the basis for identifying the candidate genes for eosinophilia.