The HIS51 monoclonal antibody specifically binds to the rat Thy-1 antigen (CD90) expressed by hematopoietic stem cells, early myeloid and erythroid cells, immature B lymphocytes in the bone marrow and peripheral lymphoid organs, thymocytes, recent thymic emigrants (a subset of CD45RC- peripheral T cells), neurons, glomerular mesangial cells, endothelium at inflammatory sites, mast cells, and bone marrow-derived dendritic cells. Rat dendritic epidermal T cells (DEC) are Thy-1-negative. CD90 is a GPI-anchored membrane glycoprotein of the Ig superfamily which is involved in signal transduction. In addition, there is evidence in the mouse that CD90 mediates adhesion of thymocytes to thymic stroma. HIS51 antibody cross-reacts with the mouse Thy-1.1 alloantigen of the AKR/J and PL strains, but not Thy-1.2 found on most strains. In the mouse, CD90 is found on thymocytes, most peripheral T lymphocytes, some intraepithelial T lymphocytes (IEL, DEC), hematopoietic stem cells, and neurons, but not B lymphocytes.