The leukocyte-common antigen (L-CA) is a family of large molecular weight glycoproteins uniquely expressed on the surface of all nucleated cells of hematopoietic origin. The glycoprotein consists of a heavily glycosylated exterior domain, a single membrane spanning region, and a large cytoplasmic domain that contains tyrosine phosphatase activity. To investigate the function of this family, we generated T cell clones that lacked L-CA (L-CA-). The expression of the ap T cell receptor, CD3, CD4, IL-2 receptor (~ 55) LFA-1, Thy-l, and Pgp-1 (CD44) was normal. The L-CA T cell clones failed to proliferate in response to antigen or cross-linked CD3; however, they could still proliferate in response to 11-2. An L-CA+ revertant was obtained and the ability to proliferate in response to antigen and cross-linked CD3 was restored. These data indicate that L-CA is required for T cells to enter into cell cycle in response to antigen.