The production of human eosinophils in vitro from normal bone marrow by using murine eosinophil differentiation factor (mEDF/interleukin 5) is described. Eosinophil production was selective and first detectable after 14 days and reached a peak between 21 and 35 days when they were the predominant cell type (41% to 89%). Until day 14, all the eosinophils were typical myelocytes, developing thereafter into metamyelocytes and mature cells. All cell types had characteristic light- and electron-microscopic features, apart from the absence of granules with crystalline cores. The eosinophils produced were readily recovered, and both immature myelocytes and mature cells were functionally active in an antibody-dependent, cell-mediated cytotoxicity assay. mEDF added into the assay enhanced the cytotoxicity but to a lower degree than previously reported for peripheral blood eosinophils, which suggests that they may be partially activated. The possibility that eosinophils could be deactivated was tested by removing mEDF from the culture medium. The eosinophils retained viability and functional activity, however, and showed no increased ability to be activated by mEDF for up to six days after removing the mEDF. The liquid culture of human bone marrow was shown to be an alternative assay for eosinophil differentiation factors to colony formation.