A new method is described for investigating the killing of Staphylococcus aureus by human phagocytic cells. The radioassay is based on the principle that only viable bacteria synthesize DNA and incorporate [3H]thymidine. Phagocytes are incubated with bacteria and then disrupted by a single freeze-thaw cycle. The uptake of tritiated thymidine by the remaining organisms is a measure of the killing ability of the phagocytes. The technique is simple, sensitive, rapid and requires small volumes of blood. It can be semiautomated and uses equipment readily available in an immunology laboratory and is therefore suitable for routine investigations of leucocyte function. It is likely that the technique could form the basis for measuring kill by phagocytes of any rapidly dividing organism. A time course and a normal range have been evaluated for the bactericidal capacity of 26 normal individuals.