Heat‐stable antigen (HSA)/mouse CD24 (formerly termed Nectadrin) is a membrane glycoprotein with an unusual structure consisting of a small protein core and extensive glycosylation. It is expressed by hematopoietic cells but not by mature T lymphocytes. HSA on accessory cells is an important costimulatory molecule required for the clonal expansion of T lymphocytes. HSA is also involved in cell‐cell adhesion events and the isolated antigen has been shown to possess self‐binding properties. In the present study we have re‐investigated the role of HSA in T cell proliferation. We find that following stimulation of T lymphocytes with concanavalin A or of CD4⁺ T lymphocytes with a combination of anti‐CD3/CD28 monoclonal antibodies (mAb) the HSA antigen is transiently expressed. The expression correlated with the appearance of CD25 and a CD2 activation epitope at the cell surface. Induction of HSA was also seen in vivo on Vβ8⁺ T lymphocytes in BALB/c mice that were injected with Staphylococcal enterotoxin B. Biosynthetic labeling and analysis of mRNA by reverse transcriptase‐polymerase chain reaction showed that HSA was synthesized by activated T lymphocytes. A combination of anti‐CD3 and mAb 79 to HSA was incapable of inducing proliferation of purified CD4⁺ T lymphocytes. However, the antibody strongly enhanced the response obtained with a combination of anti‐CD3/CD28 mAb. The augmenting effect of the HSA‐specific mAb was dose dependent. Since HSA is bound to the membrane via a glycosyl‐phosphatidylinositol (GPI) anchor and GPI‐anchored molecules have been implicated in lymphocyte activation, it is conceivable that HSA is not only a costimulatory molecule on accessory cells but is also a signaling molecule in T lymphocytes. The possibility of a homotypic HSA/HSA interaction between T lymphocytes and accessory cells is discussed.