It is known that antiinflammatory drugs inhibit the chemiluminescence produced by polymorphonuclear leukocytes (PMNL). In this respect, we studied in vitro, in standardized conditions (1.0 x 10(6) PMNL), the inhibitory effect of Voltarene on the chemiluminescence produced by PMNL from patients with rheumatic diseases, before and after treatment with the above drug. The stimulation index, as a quantitative measure of PMNL phagocytosis is higher in patients with rheumatic diseases than in controls. After treatment with Voltarene, the stimulation index further increases, especially in the patients with arthritis, suggesting a raised PMNL reactivity. A great differentiation of the in vitro inhibitory effect of Voltarene was observed especially for higher concentrations, function of age and the severity of disease. According to our results, Voltarene acts mainly by increasing the PMNL reactivity. The in vitro inhibitory effect of Voltarene on PMNL seems to be correlated with the in vivo action, the younger patients showing the greatest reactivity.