Human polymorphonuclear neutrophils (PMNs) are heterogeneous in their response to the chemotactic peptide N-formyl-methionyl-leucyl-phenylalanine (FMLP). By using a computerized imaging method for analysis of agarose chemotaxis plates, the chemotactic sensitivity of the population of PMNs at the leading front to FMLP was greater than the mean sensitivity of all migrating PMNs. When PMNs were treated with the membrane-perturbing agent butanol at concentrations up to 0.25%, chemotaxis of the fastest PMN population in response to 10(-7) mmol/L FMLP was increased to 140% of control, whereas the mean of all populations was increased to only 110% of control. These data show that the PMN population at the leading front has a different chemotactic sensitivity to FMLP. Furthermore, the susceptibility to butanol treatment of the PMN population suggests that an alteration in membrane properties may partly account for this difference.