Heat aggregated immunoglobulin G (A-IgG) of restricted size and complement solubilized tetanus toxoid (Te): human anti-Te immune complexes (IC) were sent as coded test samples to eight laboratories for quantitative assessment by different IC assay techniques including C1q solid and fluid phase binding assays, conglutinin binding assay. Raji cell test and particle counting immunoassay. In addition, samples containing the same material at concentrations communicated to the laboratories for the performance of reference curves were included. The investigators were asked to estimate the quantity of A-IgG or Te:aTe in the coded samples by reference to both their own locally produced standards and to the A-IgG and Te:aTe reference preparations of known concentrations. When calculated on the basis of locally prepared standards the range of concentrations found by the various laboratories and tests was 20-260 micrograms/ml for A-IgG (actual concentration 50 micrograms/ml) and 32-1,420 micrograms/ml for Te:aTe complexes (actual concentration 40 micrograms complexed antibody/ml). When read on the international candidate reference A-IgG preparation these ranges were 28-800 micrograms/ml and 35-800 micrograms/ml, respectively. The highest standardization efficiency was obtained when the Te:aTe reference curve was used for quantitation: the range of results obtained for A-IgG and Te:aTe coded samples being as narrow as 7-40 micrograms/ml and 34-68 micrograms/ml, respectively. Thus, when the content of the coded samples was estimated on the basis of the Te:aTe reference curves established in the laboratories a narrow clustering of the results was seen. It is proposed that the Te:aTe preparation, which has been found stable during storage for 2 years, could serve as a useful international reference preparation in the field of IC determination.