Granzyme A and granzyme B are serine proteases that mediate apoptotic signaling in cytotoxic T lymphocytes (CTL) and in natural killer (NK) cells. Both granzyme A and granzyme B are synthesized as inactive proenzymes, and they are stored within cytolytic granules and released by effector cells during degradation. This antibody should be useful for the localization of granzyme B-containing lytic granules and for the characterization of activated CTLs or NK cells.