Eosinophil chemotactic factor (ECF), previously thought to be primarily associated with human basophils and mast cells, could be released from human neutrophils (PMN) and eosinophils but not lymphocytes by the calcium ionophore A23187. Release of ECF from PMN was time and dose-dependent. Like antigen-induced, basophil-derived ECF, PMN-derived ECF had a high selectivity for eosinophils as determined by differential counts of migrating cells and by deactivation studies. Chromatographic analysis of PMN-derived ECF on Sephadex G-25 showed an elution pattern very similar to that of basophil-derived ECF. With rat peritoneal cells, it was possible to show that mast cells as well as mast cell-depleted cell preparations could be induced to release an ECF that appears to be similar or identical to human ECF. These findings suggest that ECF may play a role in inflammatory processes involving cells other than basophils and mast cells.