The authors have modified a method for determining circulating immune complexes in the complement fixation test. It is shown that with the 7% concentration of polyethylene glycol (PEG)-6000, there is a more complete precipitation of low- and medium-molecular weight immune complexes. The time and temperature of serum incubation were optimized when PEG-6000 was obtained. The use of the soluble circulating immune complexes (sCIC) prepared from human serum as a standard for the construction of a standard plot could substantially enhance the sensitivity of the method (0.325 microg/ ml). The content of circulating immune complexes was studied in donors and in patients with connective tissue dysplasia (CTD) by the improved procedure. In the group of donors, the mean level of sCIC was 0.62 +/- 0.24 mg/ml. In the CTD group, that was 2.32 +/- 0.93 mg/ml; the serum concentrations of sCIC ranging from 1.1 to 5.0 mg/ml. In the donors and patients, the detection rate of serum sCIC was 100%. The proposed method may be clinically used to measure the human serum levels of sCIC.