The proposed research is concerned with the ultrastructural detection and study of glycosyl transferases systems at the plasma membrane. Splenic lymphocytes and murine leukemic cells will be incubated in the presence of various labeled nucleotide sugars and examined using electron microscope (EM) autoradiography. Morphometric analyses will be applied to provide direct evidence for the presence or absence of ectoglycosyl transferases at the cell surface. Such evidence has already been obtained for a sialyl transferase ectoenzyme system on leukemic cells. The possible role of these enzymes or their acceptor molecules in membrane mediated cell-to-cell interactions will be indicated by analyses of grain distributions over the surface of single cells, contacting cells and cells engaged in phagocytosis. The interaction membrane macromolecules and the factors which regulate that interaction will be studied using the ectoglycosyl transferase systems. The effects of various membrane modulating drugs on the ability of the enzyme and acceptor molecules to interact with one another in the context of the plasma membrane will be studied using EM autoradiography and biochemical enzyme assays. The enzymes represent a selective means for labeling cell surface glycoproteins. The metabolic fate of membrane glycoproteins labeled in this manner will be studied to detect possible internalization or shedding processes. EM autoradiography and correlative biochemical studies will be employed to follow the radioactivity during in vitro incubations. The results should provide insight into the mechanism(s) involved in plasma membrane turnover.