The fluid gradient chamber was used to study the migration of human neutrophils in preformed gradients of N-formyl-methionyl-leucylphenylalanine. After 60 min, the chemotactic gradient Tvas replaced by a new one of identical steepness hut opposite direction, In a control group of experiments, the first gradient Tvas retained. Migration was assessed from cell distributions in filter sandwiches after 30, 60, and 90 min, Filters obtained after 5, 15, and 30 min of migration were stained with fluorescent phalloidin for microscopic evaluations of cell polarity. At 30 min most cells had polarized iu vertical directions and invaded the filters. The distance of chemotactic migration was similar during the second and the third 30-min periods (although the direction of migration was reversed in the new gradient) and significantly greater than during the first 30 min, In conclusion, the initial slow response to the chemotactic gradient represents an adaptation of the cells that later respond promptly to changes in gradient direction.