Abstract Reaction conditions have been determined for the production of soluble IgG polymers in the size range 10 S to 30 S by covalent cross-linking with glutaraldehyde. This size range is comparable with that of the immune complexes which are frequently found in the circulation of patients with certain autoimmune diseases such as rheumatoid arthritis and systemic lupus erythematosus. The yield of IgG aggregates in this size range is far greater than has been reported for cross-linking by other bifunctional reagents or for aggregation by heating. Glutaraldehyde cross-linked IgG polymers are stable and biologically reactive. They can also be labelled with fluorescein and freeze-dried with minimal loss of integrity or reactivity.