Partial bone marrow engraftment of wild-type recipient mice with marrow from MBP-1^−/−/EPX^−/− marrow donors showed that the loss of eosinophilopoiesis induced by the concomitant loss of both MBP-1 and EPX was a cell-autonomous effect. (A) Representative flow cytometric assessments of peripheral WBC populations from the resulting mixed-marrow chimeric mice are shown; they demonstrated that while non-eosinophil leukocytes displayed a 70% donor/30% recipient distribution of CD45.2/CD45.1, respectively, contributions to overall steady-state eosinophil populations in these mixed-marrow chimeric animals by donor MBP-1^−/−/EPX^−/− (CD45.2) eosinophils was limited. (B) Flow cytometric assessments of peripheral blood from mixed-marrow chimeras generated by γ irradiation of CD45.1⁺ recipient wild-type mice and subsequent adoptive transfer of either CD45.2⁺ wild-type or MBP-1^−/−/EPX^−/− donor marrow. These data showed that the non-eosinophil blood leukocytes of donor origin (% of total WBC) remained unchanged when using wild-type vs MBP-1^−/−/EPX^−/− donor marrow. However, the eosinophil populations of these mixed-marrow chimeras displayed a differential distribution of donor origin (>80% lower) when comparing recipient mice that had received wild-type vs MBP-1^−/−/EPX^−/− marrow. *P < .05.