In this study the fluid gradient chamber, a modified version of the Boydenchamber that enables mobile gradients, was used to study the migration ofhuman granulocytes in gradients of fMLP. Temporal chemotactic gradientswere created by moving density-stabilized spatial gradients at differentvelocities in relation to migrating cells. Random and directed cellmigration was quantified by applying a theoretical population distributionmodel to experimental cell distributions obtained from cell counts atdifferent depths in the filters. Rates of random and directed migrationgenerally increased with gradient velocity. At negative gradientvelocities, i.e., when the gradients were moved in a direction opposite tothat of cell migration to decrease fMLP concentration over time, random anddirected migration was inhibited. At positive gradient velocities,migration rates were not significantly different from those seen inimmobile gradients. The fact that the rate of directed migration wassmaller at negative gradient velocities indicates that negative temporalgradients reduced the average speed and/or orientation of thechemotactically migrating cells. In immobile gradients, the cells generateda small concentration increase over time when they migrated in theup-gradient direction. Consequently, a positive temporal gradient asperceived by the cells may act as a positive feedback signal to maintainchemotactic migration.