The granulocyte luminol-dependent chemiluminescence (CL) reaction is linked to the enzyme myeloperoxidase reacting with products of the respiratory burst activation. The results presented in this paper, show that the light generated in granulocytes originate both from intracellular and extracellular reactions; however, depending on the stimulus used the one or the other will dominate the activity measured. Furthermore, lysosomal fusion is proposed to be required for the intracellular CL reaction.