The capacity to solubilize immune complexes formed in vitro was measured in 25 patients with Behcet's disease. Both the level of circulating immune complexes and that of complement are increased in these patients. The solubilization capacity appeared to increase, paralleling the rise of serum complement levels in patients with Behcet's disease. Thus, increased capacity to solubilize immune complexes may be attributed to complement that is increased in these patients. The relationship between the capacity to solubilize complexes and the pathogenesis of Behcet's disease is discussed.