There is growing evidence that cytokines may regulate biological functions of mature mast cells. In the following study we have investigated the capacity of IL-4 to induce direct mediator release from rat peritoneal mast cells as well as the action of this cytokine on mast cell secretory activity. We have determined that IL-4 does not activate rat mast cells to secretion of histamine and serotonin. We have also noticed that culture of these cells for 3 or 6 hours in the presence of IL-4 does not result in the IL-2 secretion. Yet, the results of our experiments have clearly shown that IL-4 treatment for 30 min causes the decrease of spontaneous histamine release from rat mast cells (up to 57.8% of maximal value at concentration of IL-4 100 ng/ml) and this decrease is statistically significant. Moreover, we have noticed that treatment of mast cells with IL-4 significantly inhibits compound 48/80-induced histamine release. IL-4 treatment does not change the reactivity of mast cells to ConA- and anti-IgE-stimulation. These results indicate that although IL-4 does not appear to be a direct stimulus for mediator release from mature rat mast cells, it may influence mast cell secretory activity.