Functions of monocytes from the peripheral blood of 23 patients with systemic sclerosis were investigated in vitro. The yeast phagocytosis, opsonized yeast phagocytosis and binding of EA (erythrocyte-antibody) particles were found to be normal. A depressed chemotactic response was demonstrated against a zymosan-activated, complement-derived chemotactic factor. In 12 cases, monocytes were cultured for 168 hours. By the 5th and 7th days, the initially depressed chemotactic activity of monocytes returned to normal as compared to controls. This fact supports the speculation that the decreased chemotaxis cannot be caused by an intrinsic abnormality of monocytes/macrophages in systemic sclerosis.