Three widely used radioassays that depend on different principles for the measurement of circulating immune complexes (CIC) in biologic fluids are the monoclonal rheumatoid factor (mRF), Raji cell, and C1q binding tests. A comparison of the ability of these methods to measure immune complex-like material in 71 sera and 30 synovial fluids of 91 patients with rheumatoid arthritis (RA) was carried out by a group working in adjacent laboratories in a single institution. The highest number of abnormal levels in the seropositive group was detected by the C1q binding assay (91%). Levels of CIC by the mRF and Raji cell tests were elevated in 81% and 76% of the patients, respectively. The closest correlation was between the Raji and mRF tests (r = 0.44 and P = 0.002) although one depends on complement fixation and one does not. Though significant correlations between the levels of CIC determined by the C1q test and either the mRF (P = 0.2) or Raji cell (P = 0.3) assay were not found in this group, 59% of the samples had elevated levels by all three tests. The frequency of CIC in the sera of patients with seronegative RA was much lower, with the C1q test again giving the highest number of abnormal results (29% versus 16% and 12% for the Raji and mRF tests). In view of the technical problems associated with these tests, particularly lack of a uniform reliable standard, it is likely that all three tests measure the same material in most RA sera and that some of the differences observed are related to inherent variability in the tests themselves rather than intrinsic differences among the CIC detected in these samples.