In previous studies we observed that in vitro histamine release from human basophils could be dissociated from the loss of affinity of basophil granules for a cationic dye, toluidine blue. In the present study we further explored the intracellular signals leading to the decrease in toluidine blue positive basophil (TB+) numbers, with or without histamine release. Since Ca2+ mobilization is a crucial event in secretion and particularly in histamine release, we studied the role of Ca2+ in histamine release as compared to TB+ decrease. In the presence of external Ca2+ (2 mM): i) Ca2+ channel antagonists verapamil and nifedipine up to 10 microM were without effect on IgE-mediated histamine release and TB+ decrease; ii) loading of the leucocytes with Quin2 or preincubation with TMP-8, an internal Ca2+ antagonist, significantly inhibited the release of histamine and the decrease of TB+ basophils. In the absence of added external Ca2+:i) histamine release was abolished whereas the decrease of TB+ was not modified, even in the presence of EGTA;ii) the decrease of TB+ could be inhibited by prolonged EGTA preincubation, by Quin2 loading and incubation with TMB-8. We conclude that histamine release requires both external Ca2+ influx and mobilization of internal Ca2+. In contrast, no influx of external Ca2+ is required for TB+ decrease in which, however, internal Ca2+ mobilization appears to play an important role.