Immunoglobulin G (IgG) prepared from pooled human plasma contains variable amounts (up to 40%) of IgG dimer whereas IgG isolated from the plasma of a single individual is essentially monomeric. The amount of dimer increases with the number of donors contributing to the plasma pool from which the IgG is prepared. Dimerization is reversed by increasing the temp or decreasing the pH. Two intact antibody-combining sites (paratopes) are required for optimal dimer formation; the Fc region is unnecessary. These findings strongly suggest that IgG dimer consists of idiotype-anti-idiotype pairs formed between molecules of IgG from different individuals, and that a mechanism exists for suppressing idiotype-anti-idiotype formation in vivo.