We analyzed hematopoietic phenotypes in Peromyscus leucopus (PL) mice at young (2-9 months), middle (22-23 months) and old (33-46 months) ages aimed at characterizing age-associated changes in this unique rodent species. We found a significantly higher number of monocytes in old PL mice in peripheral blood, and higher proportions of CD44+ cells in blood, spleen and bone marrow in old PL mice than in middle and young counterparts. We conclude that elevated blood monocyte counts and up-regulated hematopoietic cell CD44 expression are two useful aging biomarkers for PL mice.