Although we have demonstrated that platelet activating factor (PAF) directly induces histamine release from human basophils, other studies have failed to report similar effects. In an attempt to understand the variability of these results, we examined the effect of some factors that could influence the basophils' response to PAF such as, extracellular Ca2+ and cytokines (interleukin-3 and granulocyte-macrophage colony-stimulating factor [GM-CSF]). The secretion of histamine induced by PAF was optimal when the cells were incubated in Ca2+ for 2 to 5 minutes, whereas it declined at longer time intervals up to 15 minutes. If cytochalasin B (5 micrograms/ml) was coincubated with PAF (1 mumol/L) to enhance the secretory response, histamine release was maximal at time 0 and decreased in parallel with the time of the basophils' exposure to Ca2+, like 0.1 microgram/ml anti-IgE-induced histamine secretion but unlike 1 mumol/L formyl-methionyl-leucyl-phenylalanine-induced histamine secretion. We found that there is synergy between interleukin-3 (1 to 3 ng/ml) and PAF (1 mumol/L) for secretion of histamine from human basophils (p < 0.05) and that GM-CSF (10 ng/ml) significantly (p < 0.02) potentiates the secretion of histamine activated by PAF (1 mumol/L). Our results demonstrate that: (1) the kinetics of the interaction between Ca2+ and the activation pathway that leads to histamine secretion are central events in the release reaction elicited by PAF in human basophils, and (2) interleukin-3 and GM-CSF can potentiate the secretory response of human basophils stimulated by PAF.