Recent studies have demonstrated that granzymes A and B make an important contribution to the clearance of the orthopoxvirus ectromelia, and in graft versus host disease. To test whether granzymes are generally necessary for lymphocyte-mediated cytotoxicity in vivo, we assessed the cytotoxic capacity of granzyme A and/or B-deficient lymphocytes in several perforin-dependent settings. Splenocytes and allogeneic CTL of granzyme A and/or B-deficient mice were defective for induction of DNA fragmentation, but induced significant membrane damage and target cell death. These results correlated well with the behavior of granzyme A/B-deficient CTL and NK cells in three different perforin-dependent tumor models. In a classical assay of NK cell-mediated rejection, granzyme A and/or B-deficient mice inoculated with RMA-S cells were as susceptible to tumor as wild-type mice. Perforin-deficient mice were also considerably more susceptible to tumor initiation by methylcholanthrene than granzyme A and/or B-deficient mice. Furthermore, rejection of the K1735-melanoma expressing MHC class I and II molecules was mediated by adoptively transferred H-2b anti-k CTL from immunized granzyme A and/or B-deficient mice. In summary, these data suggest that granzymes A and B are not critical for most anti-tumor effector functions of NK cells and CTL that are perforin mediated.