T cells from enlarged lymph nodes of MRL/MpJ-lpr/lpr (lpr) mice were found to express more binding sites for strongly hemagglutinating Phaseolus vulgaris agglutinin (PHA-E4) and fewer binding sites for Ricinus communis aglutinin (RCA) than those from normal MRL/MpJ-+/+ (+/+) mouse lymph node. We found that high-molecular-weight (180K-220K) glycoproteins on lpr T cells were strongly stained with these lectins on Western-blotting. These glycoproteins were found to belong to the CD45 family, by absorption with monoclonal anti-CD45 antibody. We also found that the other glycoproteins (105K and 120K glycoproteins on lpr T cells and a 105K glycoprotein on +/+ T cells) were strongly stained with the lectins which preferentially bind to mucin-type (O-linked) sugar chains on the cell surface. These glycoproteins were found to be leukosialins, by absorption with anti-leukosialin serum. From the results of the lectin-binding to these glycoproteins after sialidase treatment, CD45 antigens and leukosialin molecules on lpr T cells were found to have many more terminal alpha 2,3-linked sialic acids than those on +/+ T cells, and this fact explains why lpr T cells have more binding sites for PHA-E4 but fewer binding sites for RCA.