Plasma exchange (plasmapheresis), whereby 1-4 litres of patients' plasma are exchanged for a plasma substitute, has a therapeutic appeal dating back to medieval times. This blunderbuss therapy, removing as it does inflammatory mediators such as fibrin split products and complement breakdown products, immune complexes, antibodies, drugs and antigens, has been most clearly shown to have therapeutic effect in antibody-mediated diseases such as Goodpastures syndrome and Myasthenia Gravis. In chronic rheumatic diseases, such as systemic lupus erythematosus (SLE) and rheumatoid arthritis (RA), claims for improvement have, in the main, been based on insufficient data. More recently, a number of small, uncontrolled trials have suggested that the vasculitis and arthritis of these two diseases is improved by plasmapheresis. Of clinical interest has been the repeated observation that whereas improvement in antibody titres (e.g. DNA antibodies) is transient, clinical improvement, and a lowering of immune complex titres often last for days or even weeks. A recent observation may explain, in part, this finding. Reticulo-endothelial (RES) function, as measured by disappearance rates of heat-damaged red blood cells, is frequently impaired in SLE and RA. Plasmapheresis appears to result in improved RES function, possibly by a 'desaturating' effect on circulating and RES-bound immune complexes.