When human peripheral blood neutrophils were stimulated with various agonists which activate and/or prime neutrophils, we found that Cl- efflux was enhanced with a dramatic (50%) loss of intracellular Cl-. Interestingly, the Cl- efflux was enhanced by both agonists which induce a rapid transient increase in intracellular Ca2+ concentration ([Ca2+]i) [class I, e.g. N-formyl-methionyl-leucyl-phenylalanine (fMLP), interleukin-8 (IL8), platelet-activating factor, leukotriene B4 and C5a] and those which do not induce such an [Ca2+]i elevation [class II, e.g. tumor necrosis factor alpha (TNF) and granulocyte-macrophage colony-stimulating factor (GM-CSF)]. The time course of agonist-stimulated Cl- efflux differed depending on the agonist. Class I agonists such as IL8 and fMLP exhibited a 1 min lag phase before the onset of Cl- efflux; class II agonists such as GM-CSF and TNF displayed a 2 and 5 min lag phase, respectively. Both IL8 (class I)- and TNF (class II)-stimulated Cl- efflux exhibited similar sensitivity to inhibition by different types of ion transport inhibitors [ethacrynic acid (EA), amiloride, 4-acetamido-4'-isothiocyanatostilbene-2,2'-disulfonic acid, anthracene-9-carboxylic acid, and 4-4'-diisothiocyanatostilbene-2,2'-disulfonic acid]. On the other hand, natural Cl- efflux, which is thought to be mainly mediated by Cl-/Cl- self exchange, was not inhibited by EA (0.5 mM) or amiloride (0.3 mM). These results imply that both class I and class II agonist-stimulated Cl- efflux occurs via a common Cl- transporter which is different from that reported previously in resting human neutrophils. Although all agonists which induced a Cl- efflux also induced shape change of neutrophils, there did not appear to be a causal relationship between shape change and agonist-stimulated Cl- efflux. However, a temporal correlation was found to exist between agonist-stimulated Cl- efflux and intracellular alkalinization following agonist stimulation. Agonist-stimulated Cl- efflux therefore seems to be a common phenomenon activated by several agonists which act through different signal transduction pathways.