Interleukin-8 (IL-8) and GRO alpha are leukocyte-attracting peptides of the chemokine family. To study the priming potential of these chemokines, we measured superoxide anion production and up-regulation of N-formyl peptide receptors in human neutrophils. IL-8 and GRO alpha themselves did not stimulate production of significant amounts of superoxide anions but potentiated N-formyl peptide-induced superoxide anion production in a concentration-dependent manner. Binding measurements by flow cytometry at 37 degrees C with fluorescein-labeled N-formyl peptide revealed enhanced total N-formyl peptide binding after pretreatment of neutrophils with IL-8 and GRO alpha. Binding measurements performed at 4 degrees C indicated that the chemokines stimulated the up-regulation of N-formyl peptide receptors at the cell surface but did not alter their affinity for the ligand. This study indicates that IL-8 and GRO alpha, in addition to their known chemotactic activity, prime neutrophils for superoxide anion production, presumably by up-regulating the number of receptors for strong superoxide-anion-triggering stimuli.