To evaluate phagocytosis of granulocyte, chemiluminescence (CL) - measurements were done in whole blood specimen. By this method much less variation (22% versus 84%) could be achieved in the whole blood specimen compared with isolated granulocytes. The quenching of CL by erythrocytes, which is dependent on the quotient erythrocytes/granulocytes, could be significantly reduced by dilution (1:5,000). Optimized reaction conditions were selected (approx. 5 x more sensitive then previous protocols) to evaluate the phagocytic activity of the few remaining granulocytes (approx. 2,000/ml) with routine luminometers.