The responses of isolated human peripheral neutrophils to either simultaneous or sequential additions of two chemotactic factors were studied. Simultaneous additions of formyl-methionyl-leucyl-phenylalanine (10-100 nM) and the fifth component of complement, C5a (1-10 μl/ml), evoked partially additive responses of membrane depolarization as measured by the fluorescent dye 3,3'-dipropyl-thiocarbocyanine, a transient elevation of intracellular cyclic AMP (cAMP), and superoxide (O-2) generation as assessed by ferricytochrome c reduction. Preincubation of the cells with either formyl-methionyl-leucylphenylalanine or C5a alone caused dose-dependent inhibition of the depolarization, the cAMP increase, and O-2 release induced by a subsequent exposure to an optimal dose of the same stimulus, i.e., deactivation occurred. In contrast, when cells were treated with one chemotactic factor and then exposed to the other stimulus, the cells exhibited a normal response of peak depolarization, the rise in cAMP, and O-2 production i.e., cross-deactivation failed to occur. The results imply that deactivation of these phenomena is stimulus specific. Further, these observations are consistent with the hypothesis that cross-deactivation of chemotaxis is mediated by one or more processes that are irrelevant to O-2 generation, and that occur distal to the depolarization and cAMP steps in the sequence of neutrophil activation: possibly microtubule polymerization and orientation.