Basophils in about 15% of subjects allergic to hymenoptera venom do not release histamine in the presence of antigen. Little is known on the basophil releasability in these patients. We therefore measured maximum percent leukocyte histamine release to antigen (Vespula venom), anti-IgE and formylmethionylphenylalanine (FMP) in 39 patients allergic to wasp venom and compared our results according to basophil responsiveness to antigen. Mean maximum percent histamine release was 39,34 and 22%, respectively, for venom (100 ng/ ml), anti-IgE (0.25 µg/ml) and FMP (10^––4M). The amount of histamine specifically released by venom correlated significantly with anti-IgE but not with FMP-induced histamine release. Leukocytes were unresponsive to antigen in 10 subjects. The clinical characteristics and anaphylactic symptoms of these patients were not different from those with antigen-responsive cells. Unresponsive leukocytes responded to FMP in all and to anti-IgE in 8 of the 10 subjects. Mean anti-IgE and FMP-induced histamine release were, respectively, lower and higher than those observed with leukocytes responsive to antigen (p < 0.05). In unresponsive basophils, there was a negative correlation between maximum percent anti-IgE and FMP-induced histamine release. We confirm that basophils of a minority of the subjects allergic to Vespula venom do not release histamine in the presence of antigen. The negative correlation between anti-IgE and FMP-induced histamine release in unresponsive basophils may suggest individual differences in the ratio of FcΕRI and FMP receptors on the surface of basophils.