Polymorphonuclear leukocytes (PM Ns) isolated from peripheral blood and synovial fluid o f patients with rheumatoid arthritis and from peripheral blood o f volunteers were stimulated with 12-phorbol-13-myristate acetate (PMA). No significant differences in luminol-amplified chemiluminescence were found between different patients and control groups. However, t w o distinct patterns o f native chemiluminescence were observed. Type I showed no, or only a small, increase in native chemiluminescence with integral counts over 30min less than 3 x 105cpm, and the majority o f samples from volunteers were of this type. Type II was characterized by a burst o f native chemiluminescence starting 8 t o l 5 m i n after cell stimulation. It was found in most PMN samples from patients with rheumatoid arthritis. Integral counts over 30 min were always higher than 10'cpm and as high as 108cpm in some cases. A strong inhibition of the Type II native chemiluminescence was caused by desferal, catalase, thiourea, and glutathione. However, the luminol-amplified chemiluminescence remained unchanged or was only slightly decreased under the same experimental conditions. Sodium azide strongly inhibited both kinds of luminescence. Hydroxyl radicals, formed in a Fenton reaction, may be important intermediates in the Type II native chemiluminescence.