AbstractRabbit anti‐mouse brain (RaMBr) antiserum can induce Lyt‐1+, Lyt‐2−, T cells to proliferate and stimulates the same T cell subset to induce B cell proliferation. The aim of this report is to demonstrate that the mitogenic determinant recognized on the T cell surface by RaMBr antiserum is located on the Thy‐1 molecule expressing the products of the Thy‐1aand Thy‐1balleles. Evidence is drawn from serological and genetic experiments. The brain T cell cross‐reactive, mitogenic determinant is not expressed on Thy‐1−mutants of the BW5147 T cell lymphoma that fail to express the Thy‐1 molecule but do express other T cell surface proteins such as T‐200 and gp 69, 71. Monoclonal anti‐Thy‐1.1 and anti‐Thy‐1.2 antibodies block the binding to the appropriate T cells of the majority of the serum antibody from RaMBr antiserum. The absorption of mitogenic antibody was blocked in a similar fashion, thus demonstrating the close association of the determinant and the Thy‐1 antigen defined by monoclonal alloantibodies. The mitogenic and Thy‐1.1 determinants are probably located on the same molecule because of the data obtained with the BW5147 Thy‐1−mutants and the observation that Thy‐1aT cells, which express a lower level of surface Thy‐1 than Thy‐1bT cells, also express lower levels of the determinant recognized by RaMBr antiserum. Furthermore, in (AKR × DBA/2)F1mice (Thy‐1a/b) which express less Thy‐1.1 antigen than Thy‐1.2 at the surface, the mitogenic determinant was found to be prefentially associated with Thy‐1.2. The coordinated genetic control of the surface levels of the Thy‐1 determinant and the mitogenic determinant suggests that both determinants