Synthetic oligopeptides of appropriate structure stimulate neutrophil random locomotion, chemotaxis, lysosomal enzyme secretion, and phagocytosis. The structure-activity relationships found for enhanced migration and lysosomal enzyme secretion strongly suggest that the peptides bind to a structurally specific receptor on the neutrophil surface. It is further suggested that the binding of a peptide to the same receptor initiates all of these neutrophil functions. It is postulated that this is accomplished by the receptor-peptide combination initiating a series of parallel but coordinated and interdependent biochemical sequences leading to either microfilament or microtubule activation in addition to other processes. The various functions of the neutrophil differentially utilize the microfilament and microtubule systems.