Lewis peripheral or spleen leukocytes cultured together with (BN X Lewis) F1 hybrid cells for 6 days were reinjected into Lewis rats bearing 2 day old BN allografts. Fourteen out of 16 rats injected with 150–500 X 106 cells rejected BN allografts in an accelerated manner, i.e. 5 days after cell transfer, or 7 days after skin transplantation. Allografts on control animals were intact at this time. Controls included 86 uninjected rate, 6 injected with either Lewis or hybrid cells cultured alone, 6 injected with mixtures of normal, freshly harvested cells, and 4 given cell-free culture media. It was concluded that Lewis cells become sensitized to BN antigens when exposed to (BN X Lewis)F1 hybrid cells in culture. They appear to be capable of acquiring sensitivity in vitro as well as expressing this sensitivity when retransplanted in vivo.