The mechanisms reponsible for the deposition of circulating immune complexes have been analyzed. An active process appears to be responsible in both a laboratory model in guinea pigs and in acute immune complex disease (serum sickness) in rabbits. In rabbits, after the injection of antigen to induce serum sickness, immune complexes appear in the circulation. In addition, homocytotropic (IgE) antibody is formed which binds to the surface of basophils. Leukocyte suspensions containing these basophils, when combined with specific antigen, release a soluble factor that causes clumping of platelets and release of their vasoactive amines. An excellent correlation was found between the presence of this mechanism of release of vasoactive amine and the deposition of immune complexes in serum sickness of rabbits. Antagonists of vasoactive amines or depletion of platelets, the major circulating reservoir of these amines, suppressed the deposition of circulating immune complexes and inhibited glomerulitis and arteritis. Upon entering the walls of vessels, the complexes became lodged immune complexes, greater than 19S in size, were deposited along the membranes. The data suggest that at a time when immune complexes appear in the circulation of an immunized rabbit, vasoactive amines are released from platelets in areas where turbulence of blood occurs. Sensitized basophils participate in the release of vasoactive amines from the platelets. The amines induced increased vascular permeability which leads to deposition of large complexes from the circulation in vessel walls by a process of filtration. The deposited complexes then induce inflammatory injury.