The authors investigated the number and functions of peripheral blood monocytes in patients with mixed connective tissue disease. Moderate monocytopenia was detected in the active stage of the disease. There was a close correlation between the sensitized sheep red blood cell binding capacity of monocytes and the elevated levels of circulating immune complexes. The yeast phagocytosis of monocytes and the chemotactic activity were normal in patients with mixed connective tissue disease. C3b receptor mediated phagocytosis of monocytes decreased in the active and inactive stages of the disease. This study suggests that the decreased function of C3b receptors of monocytes is related to the pathogenesis of this disease.