Binding of the fluoresceinated chemoattractant N-formylmethionylleucylphenylalanyllysine to neutrophils was measured simultaneously with cell membrane potential by flow cytometry to determine how chemoattractant was bound by different populations of responding neutrophils. Cells exhibiting an apparent depolarization of membrane potential bound more chemoattractant than cells which did not respond or which exhibited a small hyperpolarization. Approximately 45% of the peptide bound to depolarized cells was displaced by unlabeled peptide, whereas only 3% was displaced from the nonresponding cells. By utilizing the well-known observation that extracellular fluorescein fluorescence is quenched by acid pH, it was determined that approximately one-half of the nondisplaceable peptide of both cell types was internalized, but a significant amount remained tightly bound to the extracellular membrane surface and accessible to lowering of the extracellular pH to 5.5. Treatment of cells with cytochalasin B to convert nonresponding cells to depolarizing cells also resulted in expression of displaceable binding by these cells, suggesting a close correlation between displaceable peptide binding and depolarization. Differential binding of chemoattractant among neutrophils and its modulation may account for observations of functional heterogeneity of neutrophils.