ABSTRACT A hallmark of gas gangrene (clostridial myonecrosis) pathology is a paucity of leukocytes infiltrating the necrotic tissue. The cause of this paucity most likely relates to the observation of leukocyte aggregates at the border of the area of tissue necrosis, often within the microvasculature itself. Infecting mice with genetically manipulated strains of Clostridium perfringens type A (deficient in either alpha-toxin or theta-toxin production) resulted in significantly reduced leukocyte aggregation when alpha-toxin was absent and complete abrogation of leukocyte aggregation when theta-toxin was absent. Thus, both alpha-toxin and theta-toxin are necessary for the characteristic vascular leukostasis observed in clostridial myonecrosis.