In the presence of Ca2+ and Mg2+, the chemotactic fragment of C5, the synthetic chemotactic oligopeptide formyl-methionyl-leucyl-phenyl-alanine, and the ionophore A23187 aggregated human neutrophils. Aggregation induced by the two chemotactic factors was transient and reversed within 2 to 4 minutes after exposure; aggregation induced by A23187 was sustained and continued to increase over 15 minutes. In the absence of the bivalent cations, none of these three agents aggregated the cells. If bivalent cations were added after cell contact with a chemotactic factor, aggregation was detected after, but not before, addition of the cations. Under these conditions, the magnitude of the aggregation response was sharply reduced: cells preincubated with a chemotactic factor for longer than 2 to 4 minutes aggregated minimally after addition of bivalent cations. Moreover, cells preincubated with a chemotactic factor for 4 minutes, exposed to bivalent cations, and then rechallenged with the same chemotactic factor also showed a minimal aggregation response, ie, the cells were "desensitized" to the original stimulus. However, cells desensitized to one of the chemotactic factors still aggregated prominently when exposed to the other chemotactic factor or to A23187. Cells could not be desensitized to the ionophore A23187. Desensitization of the neutrophil aggregation response closely resembles desensitization of mast cell and leukocyte degranulation. Degranulation and aggregation appear to be closely related cellular responses to immunologic stimuli. Both responses may reflect alterations in surface membrane permeability to bivalent cations and/or changes in surface membrane adhesiveness to other biologic membranes.