Establishing the appropriate yet minimal number of control mice for experiments is a critical step in experimental design. Thisdecision is particularly important regarding the study of the hematopoietic system over time, given various age-associated changes in murine hematopoietic cell populations. Here we used flow cytometry to serially monitor the frequencies of hematopoietic stem cells, common lymphoid progenitor cells, and common myeloid progenitor cells and RT-PCR assays to study the levels of Ly6a(Sca1), Slamf1, Ikzf1, and Cebpa-4 genes that control the hematopoietic process-in wildtype male and female mice with a B6SJL genetic background. These analyses revealed many differences, both at the cellular and mRNA levels, between immature and mature mice at various developmental stages. In conclusion, although it is necessary to minimize the number of mice possible insofar as possible to reduce animal use and meet animal welfare requirements, the numerous differences shown by our findings highlight the need to establish controls for every time point selected for the study of the hematopoietic system cells. This need is especially crucial when comparing immature and mature stages of mouse development.