The major alpha 2-6 sialoglycoproteins in detergent-extracts of Kurloff cells were purified by anion-exchange and Sambucus nigra agglutinin-affinity chromatographies. The similar ultrastructural localisations of (1) S. nigra agglutinin-gold conjugates and (2) acid phosphatase activities on the Kurloff body and particularly on its myelin figures indicated that the major alpha 2-6 sialoglycoproteins of the Kurloff cell had acid phosphatase activity. Two-dimensional electrophoresis showed that these tartrate-sensitive phosphatases corresponded to 2 acidic (pI 3.4-3.7) polypeptides of 36 and 34 kDa. Hydrolysis with peptide-N-glycosidases F gave a 33 kDa apoprotein rich in alanine, glutamic acid, tyrosine and lysin. A lectin-affinity study demonstrated that they contained hybrid type bisected and fucosylated N-linked oligosaccharides. Cytotoxic properties were previously attributed to Kurloff cells and other studies suggested that not only acid phosphatases but also alpha 2-6-linked sialic acid residues themselves may participate in natural killer activity.