We previously reported an in vitro bioassay permissive to eosinophil production from naive mouse bone marrow cells. Based on cytosmear analysis, recombinant murine (rm) interleukin (IL)-3 combined with granulocyte-macrophage colony-stimulating factor (GM-CSF) provided better quantitative cellular yields compared with rmIL-5. In the present study, we compared the eosinophil peroxidase (EPO) content obtained in both in vitro conditions using microtitration. Parallel with the EPO assay, the percentage of eosinophils present in cultures was determined by cytosmear analysis. Cells incubated with IL-5 displayed significantly higher EPO activity than those cultured in the presence of IL-3 and GM-CSF. At the end of the culture period, cells were further enriched in eosinophils as determined by centrifugation gradient. Enhanced amounts of cellular EPO were detected when cultures were performed in the presence of IL-5. With regard to the correlation between eosinophil maturation and EPO acquisition, eosinophil cells cultured with rmIL-5 expressed significantly more EPO activity than those generated in the presence of rmIL-3 and GM-CSF.