BACKGROUND: In many animal models that investigate the pathology of various diseases, there is a need to monitor leukocyte counts and differentials. However, various researchers use a range of different techniques in male and female laboratory animals to collect such blood variable information. These studies are then compared to one another without consideration of the possibility that different bleeding sites or techniques as well as gender may produce varying results. In light of this, the peripheral blood leukocyte counts and differentials of C57BL/6 male and female mice were determined using four blood-sampling techniques: cardiac, tail, foot, and saphenous vein punctures. METHODS: Blood smears were prepared and stained with Wright-stain for differential cell analysis. The total number of peripheral blood leukocytes was determined with the aid of a hemocytometer. Applying ANOVA and Student t-test analysis made comparisons between groups. RESULTS: The total leukocyte counts obtained using the cardiac puncture method were significantly lower as compared to the other three blood sources; saphenous, tail and foot. There were no significant differences between leukocyte counts of blood samples collected from the tail, saphenous, and foot. Additionally, no significant differences were observed in total leukocyte counts between male and female mice. Differential analysis showed lymphocytes as the predominant cell type present in the peripheral blood of both male and female mice, comprising 75-90% of the total leukocytes. While no significant differences were observed between male and female differential counts of blood collected from saphenous and tail veins, a significant difference in differential counts of blood obtained via cardiac puncture was observed between the male and female groups, suggesting the role of sex hormones. Further, of the four methods, cardiac puncture appeared to be the fastest and more reliable technique, yielding the maximum blood volume with the least amount of stress being exerted on the sampling site. CONCLUSIONS: This information suggests that in studies concerning leukocyte counts and differentials, the animal gender and the sampling site of blood collection should be kept consistent as to avoid introducing any misleading experimental variation, and that cardiac puncture is the best method of blood collection in mice.