We have examined the effect of neutrophil concentration on killing of a clinical isolate of Staphylococcus epidermidis. Human neutrophils at concentrations varying from 10⁵ to 10⁷ per ml were mixed in suspension with S. epidermidis at concentrations varying from 10³ to 10⁸ colony-forming units/ml, and the concentration of viable bacteria was assayed after various times at 37°C. The rate of bacterial killing depended on the concentration of neutrophils and not on the ratio of neutrophils to bacteria. Below a critical concentration of neutrophils, bacteria growth was greater than neutrophil killing of bacteria even when the ratio of neutrophils to bacteria was 100:1. We fitted the time course of bacterial concentration and its dependence on neutrophil concentration with an exponential function, the exponent of which is (−kp + g)t, where k is the second-order rate constant for bacterial killing, p is the neutrophil concentration, g is the first-order rate constant for bacterial growth, and t is time. We found that k ≈ 2 × 10⁻⁸ ml per neutrophil per min, and g ≈ 8 × 10⁻³/min. Only when p is greater than g/k, which we call the critical neutrophil concentration, does the bacterial concentration fall. Under optimal assay conditions, the critical neutrophil concentration was 3–4 × 10⁵ per ml, a value very close to that (≤5 × 10⁵ per ml) known to predispose humans to bacterial and fungal infections.