Long term cloned cell lines have been derived from the spleens of neonatal mice or adult mice given total lymphoid irradiation (TLI). Cell lines from both sources suppress the mixed leukocyte reaction (MLR) and the in vitro proliferation of a variety of helper T cell clones stimulated by antigen. The surface marker phenotype of the cloned suppressor cells from both sources is Thy-1+, Lyt-1-, Lyt-2-, Ig-, Ia-, and asialo-GM1. This phenotype is similar to that of cloned "natural killer" (NK) cell lines, but the suppressor cells show no NK activity in vitro. The suppressor cells have been termed "natural" suppressor (NS) cells in view of the surface marker similarities with NK cells but differences in effector function. Rat monoclonal antibodies have been made to cloned NS cells which distinguish them from murine cloned T cell lines. The proposed study aims to determine the target cell of the NS cell responder T cell vs antigen presenting cell) as well as the effect of NS cells on the generation of antigen-specific cytolytic and suppressor cells in the MLR. The mechanism of action of the NS cells will be examined to determine whether they secrete lymphokines which regulate the expression of Ia molecules on macrophages. Such lymphokines are produced by the spleens of TLI-treated mice. Monoclonal antibodies to the NS cells will be investigated to determine the nature of the surface marker identified, and to determine its tissue distribution. Finally, the lineage of the NS cells will be determined at the DNA levels by investigating the arrangement of the immunoglobulin heavy chain genes and genes which code for a T cell receptor. Rearrangements of these genetic loci are found only in the B cell lineage or T cell lineage respectively.