The lectin of the seeds of hairy vetch (Vicia villosa Roth), which selectively binds murine cytotoxic T-lymphocytes, was purified by simple affinity-chromatographic procedures on two different N-acetyl-alpha-D-galactosaminyl-carriers. The lectin thus obtained is homogeneous on polyacrylamide-gel electrophoresis both in acid and alkaline media and has a mol. wt. of approx. 120000. The lectin molecule appears to comprise four subunits of equal electrophoretic mobility, contains 4.3% of covalently bound neutral sugar and 0.72 Mn and 0.94 Zn atoms respectively. The anti-(blood-group A1) specific erythroagglutinating activity of the lectin can be detected at a limit concentration of 15 microgram/ml and is inhibitable most effectively by N-acetyl-D-galactosamine.