A new method for the evaluation of the phagocytic and bactericidal activities of neutrophil granulocytes in human disease has been developed by which the total number of viable bacteria and the number of viable intracellular bacteria in anin vitrophagocytic system are measured. Standardization of the granulocyte, bacteria and serum opsonin concentrations at their optimum has made available a sensitive method by which minor variations in neutrophil granulocyte functions can be measured. In a preliminary clinical study, patients with unrecognized abnormalities of neutrophil granulocyte functions were readily diagnosed. The results indicate that abnormal neutrophil granulocyte function is more prevalent than realized.