Objective. Estimate the contribution of monocytes/macrophages to the disease process in rheumatoid arthritis (RA), by measuring the serum levels of the leucocyte-derived granular proteins: lysozyme, myeloperoxidase (MPO), lactoferrin and human neutrophil lipocalin (HNL). Methods. Serum levels of these granular proteins were measured in patients with RA (n = 23) and in healthy controls (n = 27), and in 10 patients with RA after treatment with low-dose prednisolone. The serum levels of the granular proteins were also measured before and after treatment with metyrapone, a substance that inhibits the synthesis of cortisol in the adrenals. Results. The serum levels of lysozyme and MPO were elevated in patients with RA, while the concentrations of lactoferrin and HNL were similar in both groups. Prednisolone treatment decreased the serum concentration of lysozyme and MPO. Metyrapone did not influence the level of the granular proteins measured. Conclusions. The increased serum levels of lysozyme and MPO, but not of HNL and lactoferrin in RA could indicate a stimulated secretory activity of mononuclear phagocytes. The measurement of serum lysozyme, as an indicator of monocyte/macrophage activity, might be used to study disease activity in RA.