The immunoresistant mammary carcinoma ascites cell, TA3-St/ticol, was found to possess less cell surface sialic acid, and this of different composition, than the immunosensitive line, TA3-St, from which it was selected on the basis of weaker absorption of anti-H-2a antibody. The TA3-St/ticol cell was more pleomorphic and possessed a higher proportion of large-molecular-weight glycoprotein material (as shown by gel electrophoresis) and more filamentous cell surface material (as shown by electron microscopy) than the TA3-St cell. The more immunoresistant TA3-St/ticol/-A cell, spontaneously derived from the TA3-St/ticol cell in vivo, differed markedly in sialic acid composition from the other two lines, was more pleomorphic and possessed a greater proportion of large cell surface glycoproteins. The isolation and characterization of this material is planned. In other tumor systems, including polyoma virus-transformed cells and canine transmissible venereal tumor cells, isotopically labeled cell surface components believed to be involved in tumor rejection or in escape mechanisms will be isolated and studied for detailed chemical structures by physical, chemical, enzymatic and immunochemical techniques.