Activated polymorphonuclear leucocytes (neutrophils) can generate both intracellular and extracellular luminol dependent chemiluminescence. As luminol dependent chemiluminescence largely measures the activity of the myeloperoxidase-H2O2 system, and as the extracellular activity of this enzyme may be responsible for the tissue damage associated with inflammatory conditions such as rheumatoid arthritis, the aim of this work was to distinguish between intracellular and extracellular chemiluminescence so that the extracellular activity of this enzyme could be evaluated. Azide was used as a non-specific inhibitor of both intracellular and extracellular chemiluminescence, whereas anti-(human myeloperoxidase) IgG was used to inhibit specifically the extracellular activity of myeloperoxidase. Thus this IgG is a useful analytical tool for studying the extracellular activity of the myeloperoxidase-H2O2 system in the pathology of rheumatoid arthritis.