Skin graft rejection in ATXBM CBA mice reconstituted with naive (nonsensitized) cells was shown to be mediated predominantly by Ly-1+2- effector T cells. Thus, after the treatment of the inoculum with the monoclonal anti-Ly-1.1 antibody and complement, C57BL/6 skin grafts survived indefinitely, whereas Ly-2 antibody depletion merely delayed the onset of rejection. This showed that a Ly-1+2- precursor cell rather than a Ly-1+2+ cell was the progenitor of the Ly-1+2- graft rejection effector cell. Nevertheless, another T cell subset augmented the rejection of ski grafts and it was concluded that LY-1,2,3+ cells also provided a pool of precursor cells. Thus, it appeared that both Ly-1+ and Ly-1,2,3+ cells can function as precursor cells for the effector cells, which, as shown previously, have the ly-1+ phenotype.