Cedarlane’s anti-rat T helper cell monoclonal antibodies recognize a determinant on the majority of thymocytes (90-95%), a subset of peripheral T cells and peritoneal macrophages (1,2,3,10). The antigen recognized by this clone is a surface glycoprotein of Mr 48 000 - 52 000 and is the homologue of the human CD4 and the mouse L3/T4 antigen. Clone W3/25 labels the rat T helper subset, which mediates the helper activity for B and T cells, graft vs. host (GVH) reactivity and produces IL-2 in the mixed lymphocyte reaction (MLR) (2,4,6). Addition of W3/25 to the MLR, inhibited proliferation and blocks the production of IL-2 (4,5,6,9).