Reported herein are hematocrit and total and differential WBCs obtained from 132 clinically healthy male and female white-footed mice (Peromyscus leucopus) of four different age groups: 4, 5-8, 9-20, and >32 wk old. Minimal differences were identified between the age groups. The number and percentage of neutrophils in the 4-wk-old mice were significantly higher than those in the other groups; the relative percentage of lymphocytes in the 4-wk-old mice was significantly lower than in the 9-20-wk-old mice. The only significant gender effects identified were higher numbers of WBCs and lymphocytes in females of the 4-wk-old group and higher hematocrits in males of the 5-8- and >32-wk-old groups.