Granzyme B (GrB), a component of the cytotoxic cell granule secretion pathway, is designed to kill infected and transformed cells after intracellular delivery by the pore forming protein, perforin. The mechanism of the delivery remains speculative. In this study we tested the hypothesis that GrB possesses capacity to bind and disrupt lipid membranes. Here in comparison to previous studies that show GrB interacts with carbohydrate moieties, the protease does not bind membrane phospholipids nor has intrinsic membranolytic properties. To study the transmembrane movement of GrB, we developed a model membrane system consisting of a high-molecular weight GrB substrate encapsulated in unilamellar vesicles. Intra-vesicle proteolysis clearly requires concentrations of lytic agents (streptolysin 0, perforin or Triton X-100) that disrupt unilamellar membranes. (C) 2008 Elsevier Inc. All rights reserved.