Sialyltransferase activity and cell-cell adhesion rates of aging WI-38 cells were studied to determine the possible basis for a previously described decrease in membrane bound sialic acid and loss of proliferation of senescent cells. Ectosialyltransferase was demonstrated on the surface of both young and old WI-38 cells. The sialyltransferase assays consist of an enzyme source which is either the surface of intact cells (ectoenzyme) or a Triton X-100 cell homogenate, the nucleotide sialic acid donor (cytidine monophosphate-N-acetylneuraminic acid), and an asialo-acceptor which may be endogenous to the enzyme preparation or may be added exogenously. When sialyltransferase activity is measured in the absence of exogenous acceptors, there is a greater amount of sialic acid transferred by odl cells. However, when exogenous acceptors are provided, the amount of transfer is stimulated to a greater extent in young cells equalizing the amount of sialic acid incorporated into young and old cells. This suggests that there are fewer asialoglycoproteins and that acceptor concentration is a limiting factor in assays of young cell sialyltransferase. The end result of this may be the previously described decreased amount of membrane-bound sialic acid of old cells. A change in the adhesiveness of old cells is described which may be related to the altered cell surface.