Using PVG-RT1av1 neonatal heart grafts transplanted to the plantar space in irradiated PVG recipients, and adoptive transfer of normal and immune cells, we have analyzed the role of lymphocyte subsets in graft rejection. This assay was found to be reproducible and to show good dose-response characteristics, permitting a precise analysis of the potency of the cells transferred. Immune cells were 36 times more potent than normal cells and all of their activity was in the T cell fraction. The increase in potency of immune populations was entirely within the CD8+ population defined by the mouse monoclonal antibody MRC OX-8. The CD4+ population, defined by the antibody W3/25, in both normal and immune populations, restored first-set rejection. All the graft rejection activity of CD4+ cells could be ascribed to the MRC OX-22- subset of these cells.