TllTS, the cell surface molecule binding to the "erythrocyte receptor" of T lymphocytes: cellular distribution, purification to homogeneity and biochemical properties* T11 target structure (T11TS) is a sheep cell surface glycoprotein that binds to the E receptor of human and sheep T lymphocytes. Here we report that T l l T S has a broad tissue distribution, including mature and immature hematopoietic cells, vascular endothelium and smooth muscle. The density of TllTS expression was determined by Scatchard analysis with radiolabeled anti-T11TS monoclonal antibody. Red blood cells bound 10 000, and leukocytes bound 4000 to 23 000 antibody molecules per cell. TllTS was purified to homogeneity by immune-affinity and preparative sodium dodecyl sulfate-polyacrylamide gel electrophoresis, and some of its biochemical properties were determined. TllTS is an acidic (PI 4.5) membrane glycoprotein that binds to concanavalin A. It has 2 or 3 N-glycosidically linked carbohydrate side chains of the mature phenetype, no O-linked sugars, and an apparent mol. mass of 42 kDa (glycosylated) and 32 kDa (deglycosylated). The anti-T11TS monoclonal antibody L180/1, which blocks binding of sheep red blood cells to CD2, recognizes a protein determinant on T11TS. These findings are discussed with respect to the possible function of the CD2-T11TS system as a set of complementary cell interaction molecules involved in T cell activation.