The role of particle-bound complement proteins in the induction of noncytotoxic enzyme release from human granulocytes was investigated with the use of sera genetically deficient in complement and highly purified complement components. Release of histaminase, one of two important histamine catabolizing enzymes, and beta-glucuronidase from polymorphonuclear leukocytes was solely dependent on particle-bound C3b (the larger cleavage product of the third component of complement) when fluid-phase complement was excluded. The extent of enzyme release was a function of particle-bound C3b input, was reduced by exposing the particles to C3b inactivator, and was blocked by fluid-phase C3b. Phagocytosis of the C3b-coated particles was not required for enzyme release from neutrophils. In contrast, phagocytosis of "opsonized" particles was required for noncytotoxic release of histaminase and arylsulfatase from eosinophils; other proteins, as well as C3b, were able to opsonize particles for induction of enzyme release from eosinophils. These studies suggest a dual role for complement (particularly C3) in modulating vascular permeability phenomena, i.e., release of vasoactive mediators by the action of C3a and C5a, and release of the corresponding enzymes that inactivate the mediators by C3b.