Autoimmunity in BXSB mice is associated with a progressive increase in the number of peripheral blood mononuclear cells (PBMC). This is due to a marked rise in circulating monocytes, identified by: (a) their appearance on light and electron microscopy; (b) their surface antigenic characteristics; (c) their expression of Fc receptors; and (d) their capacity for phagocytosis. Among murine models for systemic lupus erythematosus, only the BXSB strain is characterized by monocytosis, suggesting that cells of monocytic lineage may contribute significantly to the pathogenesis of autoimmune disease in BXSB mice.