Differentiation of an eosinophilic cell line, YY-1, was induced by treatment with 0.3 mM butyric acid (BA). The effects of eosinophilotropic cytokines, such as IL-3, IL-5, and GM-CSF on BA-induced differentiation, especially in the chemotactic response to ECF, was examined. Five STO-2-derived ECFs, IL-5, C5a, and fMLP were used as eosinophil chemoattractants. The effects of the cytokines on the chemotactic response of YY-1 greatly differed according to the ECF used. It was thus suggested that heterogeneity in the chemotactic response of eosinophils depends on the cytokines which act on eosinophils during differentiation.