Five human lymphoblastoid cell lines which have become tumorigenic, as judged by transplantability into immunosuppressed mice, have been compared with earlier, non-tumorigenic stocks of the same cultures, recovered from liquid nitrogen. Analysis of the cell surface glycoproteins by SDS-PAGE, electro-transfer to cellulose nitrate and "blotting" with radioiodinated lectins, reveals an increase in the molecular weight of one band from around 190 kd to 220 kd, in each case coinciding with the acquisition of the tumorigenic phenotype. Probing the electrotransfers with a monoclonal antibody demonstrates that the altered glycoprotein is a member of the "leukocyte common antigen" family. The increase in molecular weight is due to the addition of carbohydrate residues which are removed by treatment with neuraminidase and endoglycosidase. Screening a larger number of lymphoid cell lines, including those derived from Burkitt's lymphoma, shows an excellent correlation between tumorigenicity and altered molecular weight of a leukocyte common antigen.