The presence of sialic acid in the cell surface glycoconjugates has a crucial importance in a variety of biological cell properties. It has been involved in the electrokinetic potential of the cell [l] and in the permeability of the membrane [2]. More recently, it has been related to the social life of the cell: cell-cell recognition [3], contact inhibition [4] and crypticity of immunogenic loci [5] . Moreover, changes in membrane-bound sialic acid have been reported to occur upon oncogenic transformation [6]. Further- more, the sialic acid level at the cell surface seems to be correlated to the life-time of the cell [7] and to regulate the activity of some ecto-enzymes [8]. These facts raise the question as to how the surface sialic acid level is maintained or modulated: does it need an intracellular biosynthesis of the complete glycoconjugates and their further integration in the membrane or may it be due to the activity of an ectosialyltransferase as a repair phenomenon? In fact, ectosialyltransferase activity has been widely reported [9-121 and ultrastructural evidence has been obtained [13] . In this paper, we demonstrate that rat spleen lymphocyte possesses an ectosialyltransferase which is able to transfer N-acetylneuraminic acid to its own membrane but is not able to transfer it to a macro- molecular exogenous acceptor. However, when the size of the same exogenous acceptor is reduced by proteolytic cleavage, it can reach the active site of the ecto-enzyme and is efficiently glycosylated. 2.