The effect of graft-versus-host (gvh) reactions was studied at the cellular level. It was found that splenic plaque-forming cells (pfc) and hemopoietic colony-forming units (efu) of nonirradiated adult F1 hybrid mice were inhibited following injection with parental strain lymph node cells (107). Suppression of pfc was evident 3–6 days after injection and there was only 1% of the control pfe level by 10 days. It was found that the donor lymph node cells did not contribute significantly to the remaining pfe response. Colony-forming units were depressed in the spleen by 12 clays or earlier if larger numbers of parental lymph node cells (4.7 × 107)were injected. Suppression of efu in the bone marrow was not noted during the 12-day interval studied. These results may be interpreted as evidence that pfe precursor cells and/or efu are primary targets of the gvh reaction, but other indirect mechanisms, which are discussed, are not necessarily excluded.