Human CD6 is a cell surface protein expressed by thymocytes, mature T cells, a subset of B cells and certain cells of the brain. On human T cells, CD6 has been shown to act as a co-stimulatory molecule which modulates T cell receptor (TCR)-mediated T cell activation. To study further the recently identified mouse CD6 (mCD6), we generated and characterized a set of anti-mCD6 mAb. Anti-mCD6 mAb recognizing the mCD6 scavenger receptor cysteine-rich (SRCR) extracellular domains 1 and 3 were identified. mAb against SRCR domain 3, but not domain 1, inhibited the interaction of CD6 with a recently identified ligand, activated leukocyte cell adhesion molecule (ALCAM). Immunohistochemical analysis indicated that mCD6 expression was largely localized to the T cell areas of lymphoid tissue and, as previously reported in the human, CD6 was also expressed by neurons. CD6 was highly expressed on mouse T cells isolated from the spleen, lymph node and thymus as demonstrated by two-color immunofluorescence analysis. The CD4+ and CD8+ cells in these lymphoid compartments expressed similar levels of CD6. Immunoprecipitation studies showed that mouse thymocytes predominantly express a CD6 isoform of approximately 130 kDa, while splenocytes predominantly express a CD6 isoform of approximately 100 kDa. Anti-mCD6 mAb enhanced allogeneic mixed leukocyte reactions (MLR), indicating that CD6-ALCAM interactions may regulate the proliferative capacity of T cells.