Human mononuclear leukocytes (MNL), probably OKT4-positive T cells, produced an eosinophil chemotactic factor (ECF) when they were cocultured with irradiated BALL-1, a B cell lymphoma line. Treatment of MNL, with anti-IL-2 antibody failed to suppress BALL-1-induced ECF production. Periodate-lysine-paraformaldehyde-fixed but not acetone- and ethanol-fixed BCLL induced evident ECF production. These results suggested that some cell surface molecules play a role in the induction of ECF production. Isoelectric point of BALL-1-induced ECF was around pH7, whereas that of IL-2-induced ECF was around pH 5. The molecular weight of BALL-1-induced ECF was between 10 and 30 kD. Although a combination of MoAb against IL-3, IL-5, and GM, CSF suppressed the activity of IL-2-induced ECF, it failed to suppress that of BALL-1-induced ECF. Furthermore, BALL-1-induced ECF suppressed fMLP-induced respiratory bursts of eosinophils, while IL-2-induced ECF failed. We propose that at least one reason for eosinophil infiltrate into the stroma of tumors is that the tumor cells stimulate T cells to produce BALL-1-induced ECF, and the eosinophils attracted by the ECF exhibit different functions from those by other ECF.