In this study we attempted to compare cord blood derived, in vitro differentiated eosinophils to peripheral blood eosinophils with respect to their capacity to respond to various activators and, therefore, their potential ability to contribute to an inflammatory response. The cells were compared with respect to their density, content of eosinophil peroxidase, and eosinophil-derived neurotoxin, and with respect to their responses to various activators. The in vitro cultured, cord blood derived eosinophils were distinctly lighter than the freshly isolated peripheral blood cells. This difference in cell density was reflected in a slightly reduced content of both eosinophil peroxidase (1.17 ± 0.29 compared to 2.03 ± 0.22 arbitrary units/2,000 cells) and eosinophil-derived neurotoxin (18.7 ± 4.0 vs. 26.4 ± 4.5 ng/l04 cells). We compared the cells with respect to two different activation end points; the production of activated oxygen metabolites (superoxide anion) and the secretion of cationic proteins from their granules (eosinophil peroxidase and eosinophil-derived neurotoxin). In general, these responses were either the same in the two cell populations, or they were only slightly lower in the cord blood derived cells. There were, however, a few notable exceptions. Thus the secretory responses of the cultured cells to C5a and C3a anaphylatoxins and O2––/sup> production with the chemotactic peptide, formyl-methionyl-leucyl-phenylalanine, and with aggregated IgG were consistently greater than those of the normodense eosinophils. The possible implications of these differences on the state of maturation of the in vitro differentiated eosinophils are briefly di
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