A patient with systemic lupus erythematosus (SLE) and a patient with an immune complex disease resembling Goodpasture's syndrome were treated with cyclophosphamide, prednisone and repeated plasma exchanges. Circulating immune complexes decreased, and symptoms of central nervous system disease remitted for up to 15 to 20 days after plasma exchange in the patient with SLE. In vitro lymphocyte blastogenic responses to antigens were also transiently increased on two occasions following treatment. In the second patient, decreases in circulating immune complexes and clinical improvement were ascribed chiefly to immunosuppressive drug treatment. Serum antibody to keyhole limpet hemocyanin was relatively unaffected by plasma exchange in both patients. These results suggest that plasma exchange may help to deplete circulating immune complexes or alter the equilibrium between soluble antigen and antibody which causes complexes to form and circulate. It may be less effective in reducing circulating antibody levels in patients who continue to produce new antibody.