Inbred mouse strains are routinely used as genetically defined animal models for studying a wide assortment of biological and pathological processes, including immune system function. However, no studies have presented large-scale data on the immune cell populations among the inbred strains in physiological conditions. Here we present a systematic, quantitative analysis of peripheral blood cell phenotypes of 30 mouse strains assessed by flow cytometry. This cohort of mice represents a wide range of genetic origins and includes most of the strains used in genetic, physiological, and immunological studies. We evaluated the relative percentages of peripheral blood leukocyte subtypes (lymphocytes, granulocytes, and monocytes) and lymphocyte subpopulations (CD4⁺ T, CD8⁺ T, B220⁺ B, and natural killer cells) of mature (6-mo-old) mice. Our comprehensive study demonstrated: 1) marked differences in the relative proportions of blood cell populations among the strains at this age, 2) considerable variation of each immune trait with more than twofold difference between strains with the highest and the lowest trait values, and 3) haplotype analysis revealed a strong correlation between eosinophil percentage and a single region on chromosome 14 containing two candidate genes. The strain differences described here provide important information for researchers applying immunophenotyping of peripheral blood in immunological and genetic studies. The data from this study are available as part of the Mouse Phenome Database at http://www.jax.org/phenome.