Studies continue on the role of the reticuloendothelial system in the clearance of immune complexes and in the production of autoimmune disease. Particular emphasis has been placed on Fc receptors, their ennumeration, and mechanism of action. Last year we reported the development of methods designed to determine average number of Fc receptors on peripheral blood monocytes and their average association constant. A large group of patients with autoimmune hemolytic anemia were shown to have a marked increase in their cell membrane Fc receptor number with a normal association constant of their receptor protein. On steroid therapy, the receptor number decreased toward normal. The role of glucocorticoids in receptor number and clearance rates was studied in normal volunteers. It was shown that the number of Fc receptors/cell on peripheral blood monocytes was down regulated by 4 to 5 days of 1 mg/kg Prednisone. Studies were conducted on patients with active systemic lupus erythematosus. These patients were shown to have a marked increase in peripheral blood monocyte Fc receptor number with a modest decrease inceptor association constant, both of which correlate with disease activity.