Priming effect of monocyte culture supernatant on polymorphonuclear neutrophils (PMN)-chemiluminescence (CL) was studied in patients with systemic lupus erythematosus (n = 11) and mixed connective tissue disease (n = 4). In normal controls, N-formyl-methionyl-leucyl-phenylalanine (FMLP) induced PMN-CL was enhanced when PMN were previously incubated for 15 minutes with monocyte culture supernatant (MS) or T lymphocyte culture supernatant (TS) or T lymphocyte culture supernatant (TS). The enhancing effect of MS on PMN-CL was greater than that of TS. This enhancing effect of MS was inhibited by adding of dexamethasone (1 microgram/ml) during the culture. Recombinant human TNF also enhanced PMN-CL as well as MS. When compared the enhancing effects of MS between patients and normal controls, that of patients under corticosteroid therapy (average prednisolone dose 39.5 mg/day) was reduced significantly. Thus, we concluded that the cytokines from monocyte contributed PMN phagocytosis of invading microorganisms, and that this monocyte-mediated PMN phagocytosis was suppressed partly by corticosteroids in collagen disease.