Vascular basement membrane was disrupted in the presence of polymorphonuclear leukocytes (PMN's) during two immunologic reactions: The Arthus phenomenon and the reaction to locally injected antibody to vascular basement membrane. This disruption was evidenced by (a) the inability of the basement membrane to retain circulating carbon, by (b) loss of antigenic constituents, and by (c) electron microscopic observation showing actual gaps in the structure of the vascular basement membrane. The factors within PMN's responsible for damage to isolated glomerular basement membrane in vitro were found by isolation procedures to be cathepsins D and E. Cationic proteins of PMN's were separable from the cathepsins. While inducing vascular permeability upon injection, these basic proteins failed to inflict the severe damage to the basement membrane observed in Arthus and antibasement membrane reactions. It is concluded that the full expression of these immunologic lesions requires destruction of the basement membrane possibly brought about by cathepsins D and E. Some of the physicochemical properties of these pathologically active leukocytic factors are given.