AbstractPrevious studies of human basophil mediator release have noted that the bacterial peptide fmet‐leu‐phe and the anaphylatoxin C5a induce comparable levels of histamine release while only fmet peptide induces leukotriene release. Since 5‐lipoxygenase metabolism of arachidonic acid is calcium dependent, we examined the characteristics of the human basophil [Ca++]iresponse which follows its activation by either fmet peptide or C5a. While the peak [Ca++]iresponse was essentially identical for these two stimuli, fmet peptide induced a prolonged increase in [Ca++]iwhile C5a stimulated only a transient increase in [Ca++]ithat was essentially over within 2 minutes of adding the stimulus. Simultaneous addition of EDTA with fmet peptide revealed the two phases of the [Ca++]iresponse and demonstrated that leukotriene release was dependent on an elevated [Ca++]ilevel in the 2–5 minutes following challenge. Enhancement of leukotriene release induced by C5a by agents such as staurosporine and interieukin‐3 also produced a [Ca++]ikinetic curve which resembled fmet piptide. Single cell studies of the [Ca++]iresponse could detect no subpopulations of cells which responded preferentially to fmet peptide or C5a, eliminating the possibility that the ability of fmet peptide to induce leukotriene was a result of its action on a functionally distinct population of basophils.