The prevalence of circulating immune complexes (CIC) and C4 and C3 activation products (C4 and C3 A.P.) was investigated in sera from 106 blood donors, 100 hospital staff-members, 63 patients with rheumatoid arthritis and active synovitis, and in 25 hospital staff-members who had monthly tests performed during one year. CIC were detected by means of a complement consumption test and a polyethylene glycol precipitation test. C4 and C3 A.P. were demonstrated by means of crossed immunoelectrophoresis. No significant differences in the prevalence of CIC were found between blood donors and hospital staff-members (3 and 9%, respectively) and no age and sex differences were observed. In the longitudinal study of hospital staff-members, a significantly increased incidence of CIC was found during the winter months. CIC were found in 86% of the patients with rheumatoid arthritis. C4 and C3 A.P. were found significantly more often in sera with than without CIC. The significance of CIC detection may be increased by the simultaneous demonstration of C4 and C3 A.P. and by making allowance for seasonal variations.

A systematic search for immune complexes (IC) in blood and skin revealed no correlation to IC-related disorders in 35 patients who had undergone jejunoileal bypass for obesity. Tests for cryoprecipitates and endotoxins proved negative.

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