Granzyme A, a granule‐associated serine proteinase of activated cytotoxic T cells and natural killer cells, has been reported to play a critical role in DNA fragmentation of target cells. To address the question of the biological role of granzyme A, we have now generated a granzyme A‐deficient mouse mutant by homologous recombination. Western blot analysis, enzyme assays and reverse transcription‐PCR confirmed the absence of granzyme A in activated T cells. In addition, deletion of granzyme A does not alter the expression patterns of other granule components, such as granzymes B‐G and perforin. Granzyme A‐deficient mice are healthy and show normal hematopoietic development. Most notably, their in vitro‐ and ex vivo‐derived cytotoxic T cells and natural killer cells are indistinguishable from those of normal mice in causing membrane disruption, apoptosis and DNA fragmentation in target cells. Furthermore, granzyme A‐deficient mice readily recover from both lymphocytic choriomeningitis virus and Listeria monocytogenes infections and eradicate syngeneic tumors with kinetics similar to the wild‐type strain. These results demonstrate that granzyme A does not play a primary role in cell‐mediated cytotoxicity, as has been assumed previously.