Circulating IgG immune complexes were assayed by means of agglutination inhibition of IgG-coated latex particles to human IgG by rabbit IgM antibodies. Sera from thirty patients with Behçet's syndrome, thirty patients with recurrent oral ulcers and thirty healthy control subjects were analysed. The results showed raised levels of immune complexes in 60% of patients with Behçet's syndrome and 40% of patients with recurrent oral ulcers. There were significant differences in the levels of immune complexes between the neuro-ocular and arthritic types of Behçet's syndrome as compared with the muco-cutaneous type of Behçet's syndrome or recurrent oral ulceration. A close association was found between the disease activity and the amount of immune complexes. These results suggest that immune complexes may play a part in the transition from the epithelial involvement of mucosa or cutaneous tissues to the neuro-ocular and arthritic types of Behçet's syndrome. Another factor in tissue distribution might be the size of immune complexes, as multi-focal involvement of tissues in the neuro-ocular type of Behçet's syndrome is associated with a broad range of immune complexes from 3 X 10(5) to 3 X 10(6) daltons.