The light-scattering properties of human neutrophils were compared on a cell-by-cell basis before and after stimulation with chemotactic peptide using flow cytometry. Between 20 and 180 sec after peptide addition, side (90 degrees) scatter declined by up to 4% and forward scatter increased up to 6%. Between 3 and 15 min, side scatter increased up to 15% and forward scatter decreased up to 5%. Association of a fluorescence chemoattractant with neutrophils was most rapid during the initial phase of increasing forward and decreasing side scatter, and association saturated before the maximum increase in side scatter. Evidence is presented that the observed changes in scatter were not a consequence of chemoattractant-induced cell-cell adhesion or neutrophil degranulation. Rather, the early phases of light-scattering changes are interpreted to represent membrane ruffling by the stimulated neutrophil; the later phases polarization of the neutrophil morphology.