The applicability of affinity chromatography to the isolation of Clq‐binding immune complexes (IC) in sera was explored. Purified human Clq was covalently coupled to agarose or adsorbed to IgG‐agarose resins. Sera containing preformed virus‐antibody complexes or rheumatoid arthritis (RA) sera were passed through the columns and Clq‐bound IC, eluted off with 1,4‐diaminobutan at mild basic conditions, were analysed by immunodiffusion, crossed immunoelectrophoresis, gel filtration and electron microscopy. Under conditions of antibody treatment which caused almost 100 per cent inhibition of virus plaque formation, about 30 per cent of formed14C‐labelled equine arteritis virus‐antibody complexes was bound specifically to and desorbed from Clq‐IgG agarose columns. Studies with RA‐sera indicated the presence of both IgM‐IgG and intermediate size IgG, Clq‐binding, complexes in 3 out of 5 tested seropositive sera. In two sera only intermediate size IC were demonstrable. The results obtained in these two IC model systems suggested that the described methods could be useful for isolation of Clq