CD22 is a B cell lineage restricted cell-surface adhesion glycoprotein which recognizes ligands on human T and B cells and cell lines. A soluble recombinant form of human CD22 (hCD22Rg) has been used to identify and characterize CD22-specific ligands on human T cells, one of which has been shown to be the receptor-linked phosphotyrosine phosphatase CD45. Because CD45 plays a pivotal role in lymphocyte activation, we assessed whether human CD22 might display cross-species reactivity with CD45. In the study presented here we demonstrate that human CD22Rg recognizes several murine cell-surface sialoglycoproteins, including CD45, containing sialic acid in alpha 2,6 linkage. Furthermore, hCD22Rg recognizes different ligands on functionally distinct T helper-cell subpopulations and selectively binds medullary thymocytes in vivo. Our results confirm and extend previous observations that CD22 is a sialic acid-binding lectin which interacts with CD45 and other glycoproteins capable of presenting alpha 2,6-linked sialic acid in a manner that promotes high affinity binding. The cross-species reactivity of CD22 with its ligands underscores the potential physiologic importance of CD22-mediated lymphocyte interactions