Accumulating evidence suggests a link between immediate hypersensitivity and cellular immunity. In this study, we examined the effect of interleukin 2 (IL-2) on basophil histamine release. Histamine-releasing activity of IL-2 was very weak with % histamine release of 2.9 +/- 1.3 (mean +/- SEM, n = 9) at 1:12 dilution. IL-2 at 1:1200 dilution slightly inhibited anti-IgE-induced histamine release by 22.4 +/- 18.6% (P greater than 0.05). There was a significant potentiation of release at 1:12 dilution of IL-2 with % enhancement of 78.7 +/- 42.2 (P less than 0.05). IL-2 enhanced the calcium ionophore A23187-induced histamine release in a dose-dependent fashion. IL-2 at 1:12 dilution significantly potentiated release by 28.8 +/- 6.3% (P less than 0.05). There was a slight suppression of formyl-methionyl-leucyl-phenylalanine (FMLP)-induced histamine release at 1:1200 dilution with % inhibition of 23.4 +/- 7.4 (P greater than 0.05). At 1:12 dilution, IL-2 significantly potentiated FMLP-induced release by 73.7 +/- 41.6% (P less than 0.05). Recombinant IL-2 (RIL-2) augmented anti-IgE-induced histamine release with a significant enhancement at 200 units/ml. Conventional IL-2 was more potent than RIL-2 in enhancing release. These results indicate that IL-2 enhances basophil histamine release and some part of the effect of IL-2 on basophils is derived from other factors contained in conventional IL-2.