ABSTRACTThe cell surface chemistry ofPhysarum polycephalummyxamoeba strains RSD4 and MA 185, both capable of axenic growth, was studied. Myxamoebae were either washed with a low molarity phosphate/salt buffer or were left unwashed. The freeze-fractured, deep-etched cell surface of RSD4 cells was normally smooth, but after washing in buffer it was fibrous and rough. Similarly treated cells of strain MA 185 were smooth under both circumstances. Several lectins were tested for their ability to bind to the cell surfaces of both strains. While concanavalin A bound only to washed cells of RSD4, it labeled both washed and unwashed cells of MA 185. Wheat germ agglutinin bound in a patchy pattern to washed cells of both strains. A rosette test determined the ability of washed and unwashed myxamoebae of both strains to bind erythrocytes. Sheep red blood cells bound to the surface of unwashed cells of strain MA 185. Analysis of the material washed from the surface of strain RSD4 cells with the phosphate/salt buffer indicated both protein and carbohydrate components. Between 16 and 20 polypeptides, indicated by Polyacrylamide gel electrophoresis, were found in ethanol and ammonium sulfate precipitates of the cell surface material, in addition to four concanavalin A binding glycoproteins.