The mixed antiglobulin reaction and the formation of nonimmune rosettes with guinea-pig red blood cells (RBC) distinguished feline B and T cells, respectively. In a cat with thymic lymphosarcoma, the cells reacting in these tests formed separate, nonoverlapping populations. The malignant cells were large lymphoblasts replacing the normal thymus and infiltrating local lymph nodes, where they localized only in the paracortical, i.e., thymus-dependent areas. Cells from the nodes could therefore be identified as malignant or normal by their size. The mixed antiglobulin reaction showed that the malignant cells did not carry the surface Ig characteristic of B cells, whereas these malignant cells formed nonimmune rosettes with guinea-pig RBC. Among lymph node cells, most surviving normal and small lymphocytes, from outside the thymus-dependent areas, reacted as B cells. The morphologic evidence therefore corroborated the test results, which indicated that the formation of rosettes with guinea-pig RBC seems a reliable means for the demonstration of T cells in the cat.