A variation of the method of Preisig and Hitzig (18) for the determination of phagocytosis and NBT-reduction of polymorphonuclear granulocytes and monocytes in the peripheral blood is described. Contact with infectious cultures of Candida albicans is avoided by the use of baker's yeast for phagocytosis. Methylgreen is replaced by nuclear fast red for staining. The dye solution can be used over a longer period, the stained slides do not lose their colour, the contrast with the reduced blue NBT is more intense. The application of the method is demonstrated by the results of the test in the family of a boy suffering from CGD.