Systemic lupus erythematosus (SLE) is characterized by multiple defects affecting B and T lymphocyte cell function. In view of the influence that monocytes may have on these functions, we have examined monocyte function in patients with SLE and normals. Monocytes were examined as to number, their ability to modulate IgG synthesis, phagocytose yeast particles, and acid phosphatase activity. Patients had more monocytes and mononuclear cells made more IgG, but these factors appear to be independent of each other. Monocytes from lupus patients are less phagocytic but show similar levels of acid phosphatase activity to normal controls. Normal, but not lupus monocytes, show enhanced phagocytosis following treatment with LPS. The effect of monocyte stimulation on acid phosphatase activity is the same in lupus and normal monocytes. Taken together, these observations suggest that the increased numbers of monocytes in lupus patients and some subtle functional differences in these cells may be important in the progression of this disease.