The mechanism of neutrophil activation by the chemotactic peptide formyl-methionyl-leucyl-phenylalanine (FMLP) has been studied by pretreatment of human neutrophils with pertussis toxin. Upon stimulation with FMLP, the cytosolic-free calcium concentration, [Ca2+]i, is increased both by stimulation of calcium influx and mobilization of cellular calcium. We have measured [Ca2+]i as well as the generation of the phospholipid breakdown product inositol trisphosphate (IP3), which is thought to mediate Ca2+ mobilization. As the phosphoinositide pool in human neutrophils is difficult to prelabel with [3H]myoinositol, experiments were also carried out in the cultured human promyelocytic leukemia cell line HL-60 after differentiation with dimethylsulfoxide. Pertussis toxin pretreatment of both cell types inhibited FMLP stimulated membrane depolarization, exocytosis, and superoxide production in a dose-dependent manner. This toxin effect was selective for the receptor agonist, since stimulation of these parameters by two substances bypassing the transduction mechanism, the calcium ionophore ionomycin and the phorbolester phorbol myristate acetate, were unaffected. Rises in [Ca2+]i, as well as generation of IP3 in response to FMLP, were inhibited in parallel; for the inhibition of functional responses, slightly lower toxin concentrations were required. The attentuation of the [Ca2+]i rise was more marked in the absence of extracellular calcium, i.e., when the rise is due only to calcium mobilization. The results provide evidence that phospholipase C stimulation by FMLP resulting in IP3 generation is involved in the signal transduction mechanism. Coupling of FMLP receptor occupancy to phospholipase C activation is sensitive to pertussis toxin, suggesting the involvement of a GTP binding protein (N protein), which has been shown to be a pertussis toxin substrate. The parallel changes in [Ca2+]i and IP3 further support the hypothesis that IP3 is the calcium-mobilizing mediator in FMLP-activated cells.