 fMLF and other "N"-formylated oligopeptides were found to be similarly active in human neutrophils.The high degree of structural specificity of a broad series of formylated peptides in stimulating these neutrophil responses, the specific binding of "N"-formylated oligopeptides to neutrophils with affinities that paralleled their stimulating potencies, the ability of "t"-carbobenzoxy-phenylalanyl-methionine to bind to but not stimulate neutrophils and thereby to block the neutrophil binding and stimulating activity of N-formylated oligopeptides, and the ability of the formylated oligopeptides to desensitize (i.e.render unresponsive) neutrophil functional responses to themselves but have no or a lesser ability to desensitize to a range of other chemotactic stimuli provided strong suggestive evidence that the formylated peptides acted on cells through a common, dedicated receptor system that differed from other chemotactic factor receptors.