SUMMARYThe possible involvement of a suppressor cell mechanism in neonatally induced allograft tolerance was tested in two different mouse‐strain combinations. In an H‐2 incompatible donor‐recipient combination presented by the congeneic strains B10.D2 and B10.D2(M504), adoptive transfer of tolerance with 5 × 107spleen cells to syngeneic sublethally (400 R) irradiated recipients was successful resulting in a markedly prolonged test graft survival or even take. The result of tests for negative tolerance (which should be susceptible to abolition by means of cells from normally reactive donors) did not indicate that neonatally induced tolerance across the given barrier rested on this principle. The suppressor mechanism seemed to be involved in neonatally induced allograft tolerance also in the other strain combination (B10