The role of the two main serum opsonins (IgG and C3b) in the induction of polymorphonuclear leukocyte chemiluminescence was studied in a group of psoriatic patients. Chemiluminescence was stimulated with zymosan opsonized by fresh plasma (IgG- and C3b-dependent chemiluminescence) or by complement-depleted plasma (IgG-dependent chemiluminescence). While C3b-dependent chemiluminescence was similar in patients with chronic or active forms of psoriasis, IgG-dependent chemiluminescence was significantly increased in patients with active disease. However, FcR-III expression, evaluated by means of flow cytofluorimetry, was similar in the different groups of patients studied. The discrepancy between Fc-receptor (CD16) expression and IgG-dependent chemiluminescence is, therefore, indicative of modifications that occur in psoriatic neutrophils that do not involve FcIII-receptor expression.