The biological role of granzyme K, a serine protease of cytotoxic T lymphocytes (CTL), is controversial. It has been reported to induce perforin-mediated cell death in vitro, but is also reported to be non-cytotoxic and to operate in inflammatory processes. To elucidate the biological role of this protease we have deleted the granzyme K gene in mice (mutant allele: Gzmktm1.1Pib; MGI:5636646). Gzmk-/- mice are healthy, anatomically normal, fecund, and show normal hematopoietic development. Gzmk-/- mice readily recover from Lymphocytic choriomeningitis virus and mouse pox Ectromelia virus infection. Ex vivo, virus-specific granzyme K – deficient CTL are indistinguishable from those of wild type mice in apoptosis induction of target cells. These data suggest that granzyme K does not play an essential role in viral immunity or cytotoxicity. Our granzyme K knockout line completes the collection of mouse models for the human granzymes, and will further our understanding of their biological roles and relationships.