To test the granule exocytosis model for lymphocyte cytotoxicity, we have expressed the gene for the cytotoxic lymphocyte granule protein cytolysin (perforin) in the noncytotoxic rat basophilic leukemia (ML) cell line, which undergoes granule exocytosis when its high affinity IgE receptor is cross-linked. Homogenates of RBL-cytolysln (RBL-cy) transfectants showed a calcium-dependent hemolytic activity in dense granule fractions, demonstrating that the expressed cytolysin protein was correctly targeted to secretory granules. RBL-cy transfectants showed a potent and calciumdependent cytolytic activity against IgEtoated RBCs, while the parental RBL line was not cytotoxic under these conditions. RBL-cy cells did not lyse non-IgE-coated RBCs copelleted with targets: this sparlng of ‘irr nocent bystanders” parallels cytotoxic T lymphocyte lysis and suggests a polarized secretion of cytolysln. In contrast to RBC targets, IgE-coated tumor cells were much less sensitive to lysis by RBL-cy transfectants.