A quantitative assay for determining the polymorphonuclear neutrophil (PMN) stimulation is described. The method is based on the inactivation of lambda vector phages that occurs after a brief exposure to stimulated PMNs. The determination of the number of residual plaque-forming units on the appropriate bacterial host allows a reproducible and sensitive quantitative assay for measuring the stimulation level of the PMN. In comparison with other methods that employ bacteria or eukaryotic cells, this assay provides several advantages and can be used for investigating the biochemical and physiological processes responsible for PMN stimulation.