The N-linked oligosaccharides of neural cell adhesion molecule and the rat brain voltage-dependent sodium channel alpha subunit are specifically modified by alpha 2, 8-polysialic acid chains. Until now, this carbohydrate modification has been observed only on these two proteins in mammalian cells. We have identified 180-260 kDa proteins in RBL rat basophilic leukemia cells and MCF7 human breast cancer cells that are modified by alpha 2, 8-polysialylated oligosaccharides. Immunofluorescence microscopy and Northern analysis confirmed that these proteins are neither the neural cell adhesion molecule nor the sodium channel alpha subunit. The presence of authentic alpha 2, 8-polysialic acid on the basophilic leukemia and breast cancer proteins was confirmed by the elimination of anti-polysialic acid antibody staining after treatment with the alpha 2, 8-polysialic acid-specific endo-N-acetylneuraminidase. The failure of peptide N-glycosidase F to completely remove alpha 2, 8-polysialic acid bearing oligosaccharides from the RBL protein, and the sensitivity of these oligosaccharides to beta-elimination, suggests that alpha 2, 8-polysialic acid may be found on O-linked oligosaccharides. This identification of new alpha 2, 8-polysialylated proteins in RBL basophilic leukemia and MCF7 breast cancer cells suggests that alpha 2, 8-polysialylation of glycoproteins may be more widespread than originally believed, especially in cancer cells.