We have previously described a monoclonal antibody, 984, which specifically recognizes murine CD8+ suppressor T cells (Ts) but not CD8+ cytolytic T lymphocytes (CTLs). Removal of 984+ cells abrogates the suppressive effect of CD8+ Ts generated either in vivo or in vitro while having no effect upon CTL. In this report, the molecules recognized by 984 are identified as 2-6 sialylated neolacto series gangliosides, which are members of the newly defined CD75s cluster. We proceed to demonstrate that like 984, a separate anti-CD75s antibody (CRIS-4), recognizes primary CD8+ Ts cells. In addition, the 2,6 sialyltransferase responsible for the synthesis of the 984 epitope is identified, allowing the manipulation and study of the regulation of this epitope. This is the first report of CD75s on murine cells and the first report that delineates lymphocyte function based upon CD75s expression. In addition to contributing to the growing body of evidence that lineage dependent gangliosides are expressed by T lymphocytes, these findings suggest that CD8+ CD75s+ T lymphocytes represent a functionally distinct subset of CD8+ T cells with negative regulatory function.