The present study examined the properties of NK activity in Yorkshire swine. The results support other porcine studies which indicate the swine NK system has both similarities and differences to this system in other species. Profiles of NK activity indicated swine NK cells are highly reactive against the YAC-1 lymphoma, the K-562 myeloid leukemia, the P-815 mastocytoma, and the TU-5 virally transformed fibroblast. In contrast, the MOLT-4 and SB leukemias are NK resistant. Kinetic studies indicated that in Yorkshire swine, NK lysis begins 6 h after mixing effectors and targets. The kinetics of the lytic reaction differ both from other breeds of swine and from other species, where cytotoxicity is readily measured in 4-h assays. The delayed lysis was not due to delayed target cell recognition, because Yorkshire swine NK cells are rapidly bound to tumor targets. The delayed lysis seems to be due to a refractoriness in the NK lytic mechanism. This delay may relate to the morphologic finding that the target-binding cell in Yorkshire swine appeared quite different from the large granular lymphocyte (LGL) reported as the NK effector in humans and rodents. Indeed, in light microscopic studies the typical tumor-binding cell in Yorkshire swine is a small, apparently nongranular, lymphocyte. Analysis of NK activity at the single cell level was performed with single effector-tumor conjugates immobilized in agarose. Generally, lysis by target binders paralleled sensitivity to lysis in 51Cr release tests, indicating lysis in agarose may be used as an NK index in swine. Like other species, swine NK cells were found to be nonadherent lymphocytes with a characteristic tissue distribution. Peripheral blood and spleen had the highest levels of NK activity. Lymph node cells displayed a small amount of NK activity which was limited to the YAC-1 target, while thymocytes showed no appreciable NK activity against any of the cell lines tested.