Effects of the farnesylcysteine mimetic, farnesylthiosalicylate on the activation of myeloid cells were studied. In dimethyl-sulfoxide-differentiated HL60 cells and in human neutrophils farnesylthiosalicylate (G20 pM) dose-dependently elevated cytosolic Ca" concentrations, suggesting phospholipase-c-mediated release of the ion from intracellular stores. In human neutrophils, in addition to the production of inositol trisphosphate, farnesylthiosalicylate induced activation of the NADPH oxidase and translocation of the cytosolic oxidase components p47-phox and p67-phox to the membrane. The calcium signal, inositol-trisphosphate production and superoxide generation elicited by farnesylthiosalicylate were partially blocked by treatment of the cells with pertussis toxin, consistent with participation of pertussistoxin-sensitive and pertussis-toxin-resistant elements. In HL60 cells, farnesylthiosalicylate ( c 2 0 pM) did not activate NADPH oxidase but dose-dependently augmented PMA-elicited activity of the enzyme. This effect was resistant to pertussis-toxin treatment. In vitro augmentation of PKC-mediated phosphorylation of histone and cytosolic p47-phox by farnesylthiosalicylate and the finding that downregulation of PKC abrogated potentiation of NADPH oxidase activity by farnesylthiosalicylate were compatible with the involvement of PKC in the response of HL60 cells to farnesylthiosalicylate. It is suggested that the effects of farnesylthiosalicylate on myeloid cells reflect interaction of the analog with prenylcysteine-docking sites on cellular signaling elements.Keywords: neutrophil ; HL60 cells ; NADPH oxidase ; farnesylcysteine; farnesylthiosalicylate.Neutrophils constitute the first line of defense against invading microorganisms. They respond to exogenous signals by chemotaxis, activation of the respiratory-burst oxidase, exocytosis of their cytoplasmic granules, and phagocytosis of foreign particles. Studies of responses of neutrophils to extracellular stimuli contributed to a better understanding of diverse signal-transduction pathways and their elements. In many studies, the promyelocytic leukemia cell line HL60 (Collins, 1987) was employed as a model of neutrophils. HL60 cells induced to myelocytic differentiation by dimethyl sulfoxide (Me,SO) or retinoic acid acquire most of the features of circulating blood neutrophils.Activation of the neutrophil NADPH oxidase (Morel et al., 1991 ;Chanock et al., 1994) in response to exogenous stimuli is characterized by a rapid onset and is amenable to easy experimental analysis. The release of superoxide radicals reflects assembly at the cell membrane of the multicomponent enzymatic