A study was made of the effect of selective plasmapheresis (SPP) on the concentration of circulating immune complexes (CIC). The method is based on precipitation at 4 degrees C of plasma fibronectin and associated macromolecular complexes by means of heparin. Sterile plasma of the patient is separated from the precipitate, frozen and kept at -20 degrees C till the next plasmapheresis during which it is returned to the patient instead of the donor's one. All 15 patients examined were exposed to 6 SPP with an interval made every 2 to 5 days. Six patients were diagnosed to have rheumatoid arthritis, 2 systemic lupus erythematosus, 3 hemorrhagic microthrombovasculitis, and 4 multiple sclerosis. The concentration of CIC was measured by precipitation with 3.5% polyethylene glycol before and after SPP, in some cases between sessions. All the patients with rheumatoid arthritis and systemic lupus erythematosus and 2 out of the 3 patients with hemorrhagic vasculitis showed an elevated content of CIC (greater than 0.150 Units OD). The CIC content appeared normal in all the patients with multiple sclerosis. After SPP 4 patients manifested a reduction in the CIC concentration, whereas in 6 it returned to normal. Such a time course correlated with the improvement of other clinical and laboratory findings. It was established that after the first session of SPP the CIC content sharply declined followed by a gradual increment and exceeded the initial values toward the beginning of the second session. After the second SPP the patients manifested the same tendencies. The CIC content reached a maximum by the third SPP and then fell from session to session.(ABSTRACT TRUNCATED AT 250 WORDS)