Peripheral blood leucocytes from normal, non-immunized adult rats recognize syngeneic and allogeneic tissue antigens, and this recognition can be detected by the tube leucocyte adherence inhibition assay. We show that PBL from newborn rats have the same and even a stronger ability for recognition and that the recognition is preserved in rats bearing tolerated skin allografts after neonatal induction of transplantation tolerance. Testing of tissue extracts prepared from the congenic strain of rats suggested that PBL from normal animals recognize histocompatibility antigens.