HS1, an intracellular protein expressed specifically in hematopoietic cells, is rapidly tyrosine phosphorylated after cross‐linking of antigen receptors on B and T lymphocytes, implicating involvement of this molecule in the signal transduction pathways from the antigen receptors as a substrate of membrane‐associated tyrosine kinase(s). The development of lymphoid cells in HS1‐deficient mice, generated through gene targeting, appeared normal. However, antibody production to T‐independent antigen and proliferative responses of splenic B and T cells after cross‐linking of the antigen receptors were impaired in these mutant mice. Furthermore, B cells in the peritoneal cavity of the mutant mice were resistant to multivalent cross‐linking of the antigen receptor, which causes apoptosis of such cells in normal mice. Crossing the HS1‐deficient mice with the mice harboring transgenes encoding alpha and beta chains of T‐cell antigen receptor against a male H‐Y antigen resulted in a progeny that demonstrated a significantly impaired ability of thymic negative selection. These results indicate that HS1 is a novel molecule involved in the antigen‐receptor‐derived signaling pathways and plays important roles not only in clonal expansion, but also in clonal deletion of B and T cells.