The quantitative capacity of normal human neutrophils to ingest and kill increasing multiples of bacteria up to a level of saturation was evaluated. The in vitro system, a modification of classic bactericidal assay, employed carefully controlled ratios of bacteria per neutrophil between 1.25 and 400:1 to stress neutrophil function. Morphologic examination and measurement of bacterial survival by quantitative plate culturing indicated that at a challenge of about 100 Staphylococcus aureus per neutrophil bactericidal activity reached its peak with each average cell killing 48 bacteria. The efficiency of bactericidal function fell as the challenge was increased while ingestion capabilities kept pace with the numbers of bacteria presented to the neutrophils. Various key test parameters were evaluated from which evolved a system which provides a highly reproducible, statistically based, method for clinical assessment of subtle or partial neutrophil dysfunctions which may escape detection by the currently employed techniques of bactericidal assay.