Neutrophils stimulated by the chemotactic factor formyl-methionyl-leucyl-phenyl-alanine (FMLP) undergo a transient change in surface properties that permits the cells to adhere more readily to surfaces and to each other. This transient change can be monitored by light scattering as stimulated neutrophils form aggregates while stirred in a platelet aggregometer. Maximum change in light scattering occurs within 1 min and correlates with an increase in the percentage of cells that are in aggregates of four or more cells and a decrease in the percentage of single cells. With time (3-5 min), small aggregates disappear and single cells reappear. The transient change in adhesiveness is accompanied by a persistent change in cell shape; the cells become polarized and protrude ruffles from one sector of the cell surface. During aggregation the cells adhere to one another with smooth sides together and ruffles pointed outward. During disaggregation the cells dissociate laterally with the simultaneous internalization of membrane in the region opposite the ruffles. Particle bound to the surface by charge (thorotrast, cationized ferritin) are concentrated and internalized in this region. The change in cell shape from round to ruffled occurs within seconds, suggesting that membrane is added to the cell surface from an intracellular store. We therefore quantified surface membrane by electron microscopy morphometry and measured a 25% increase within 10 s of adding FMLP. The source of new membrane appeared to be the specific granule membrane since the kinetics of granule discharge (between 30% and 50% of all release occurs in the first 10 s) correlate with the appearance of new membrane. Furthermore, the amount of membrane that appears at the cell surface at 10 s correlates with that lost from intracellular granules in that time. Chemotaxin-induced aggregation thus begins with granule discharge and membrane addition followed by protrusion of ruffles. Adherence is maximal at 60 s and the gradual loss of adhesiveness that follows is associated with uropod formation and enhanced endocytic activity.