Immune complexes consisting of rabbit antibody to bovine serum albumin (BSA) have been made up at 1×, 3×, 6×, 8×, and 20× antigen equivalence. The complement fixing activity of these complexes is inversely proportional to the amount of antigen present in the complexes, and, as expected, solubility of the complexes progressively increases with increasing amounts of antigen. The ability of these complexes to induce acute pulmonary injury and inflammatory responses has been quantitatively assessed. Complexes preformed at antigen equivalence are the most damaging to lung, correlating with their complement fixing activity. When the antigen concentration in the complexes is increased 3 to 6 times beyond the point of equivalence, the phlogistic activity of the complexes drops off rapidly, as demonstrated by a sharp decline in the changes in vascular permeability, hemorrhage, and morphologic evidence of inflammation. These studies provide the first evidence that changing the physicochemical parameters of preformed immune complexes by simply altering the ratio of antigen to antibody can dramatically alter the phlogistic properties of immune complexes for pulmonary tissue.