In previous reports we have shown that phytohemagglutinin (PHA)-activated human lymphocytes had positive inotropic effects on spontaneously beating isolated rat atria. In this study, we demonstrated that the stimulatory effect on heart contractility induced by lymphocytes was linked to early events of lymphocyte activation by lectins. Active soluble factors were gradually released to the fluid phase. Similar results were obtained with both mitogenic (PHA) and nonmitogenic (WGA) lectins indicating that the stimulatory action of activated lymphocytes did not require cell division. Absence of Ca2+ inhibited both the generation of the stimulatory activity and lymphocyte proliferation. In contrast, verapamil, dexamethasone and low concentrations of cycloheximide eliminated only the appearance of the stimulatory effect.