A widely used murine monoclonal antibody (mAb) to the Thy-1.2 allele and a highly absorbed xenoantiserum to hamster thymocytes bind to all thymocytes and a subpopulation of peripheral lymphoid cells of the Syrian hamster (Mesocricetus auratus). The mAb appears to detect the hamster Thy-1 homologue because hamster and mouse brain tissue each completely absorb the reactivity to hamster lymphocytes. Previous studies suggested that anti-Thy-1.2 identifies hamster T lymphocytes exclusively. Fluorescence analysis, however, demonstrates a large overlap of Thy-1.2+ and slg+ lymph node and spleen cells. Moreover, highly enriched (greater than 97%) Thy-1+ lymph node cells are greater than 50% slg+, give enhanced proliferative responses to LPS and anti-Ig in vitro, and reexpress normal amounts of slg 18 to 24 hr after Ig is removed from the cell surface. Thus, the hamster Thy-1 homologue is distributed not only on T cells but also on the majority of resting, peripheral B cells, and therefore cannot be utilized adequately as a T cell-specific marker in this species.