BH3-mimetics are small molecule inhibitors that neutralize the function of anti-apoptotic BCL-2 family members. BH3-mimetics have recently gained a lot of popularity in oncology because of their success in cancer treatment. However, BH3-mimetics might have a broader clinical application. Here, we established an ex vivo flow cytometric assay allowing the comparison of the impact of BH3-mimetics (ABT-199, ABT-263, WEHI-539, and S63845) on leukocyte populations of both, healthy human subjects and C57BL/6 J wild type mice. BH3-mimetics were added to freshly drawn blood that was diluted 1/2 in cell medium, and BH3-mimetics-mediated impact on leukocyte count was assessed by flow cytometry. Our results demonstrate that responses towards 1μM of BH3-mimetics can be identical as well as considerably different in leukocytes of humans and mice. For instance, the inhibition of BCL-2 by ABT-199 caused cell death in all types of lymphocytes in mice but was exclusively specific for B cells in humans. Moreover, inhibition of BCL-XL by WEHI-539 affected solely mouse leukocytes while targeting MCL-1 by S63845 resulted in efficient induction of cell death in human neutrophils but not in their mouse counterparts. Our ex vivo assay enables initial identification of analogies and differences between human and mouse leukocytes in response towards BH3-mimetics.