1. A simple quantitative test has been developed to investigate phagocyte function. 2. This test is performed by the addition of Nitroblue Tetrazolium to whole blood, followed by the isolation of leucocytes on a column of nylon wool. Dye reduction by phagocytes is apparent as a blue coloration of the column due to the formation of formazan. The formazan can be extracted from the column and measured spectrophotometrically. 3. The formation of formazan was found to be directly related to the number of phagocytes in blood. 4. Two patients with chronic granulomatous disease gave abnormal results, suggesting that the test procedure may be of value as a screening procedure for this disease.