Carbohydrate compositions of chloroform: methanol-soluble and -insoluble complex polysaccharides have been studied in cell lines derived from BALB/c mice. The cells used in these studies include normal cells, spontaneous and viral transformants that cause tumors that regress, and spontaneous and viral transformants that cause progressively growing tumors that kill immunocompetent BALB/c mice. The carbohydrates were determined by gas-liquid chromatography. Some of the transformed cell lines compared with normal cells have altered carbohydrate compositions, including decreased sialic acid levels and decreased N-acetylgalactosamine in chloroform: methanol-soluble material. Significant decreases in total carbohydrates of chloroform: methanol-soluble material were also observed in some of the transformed cells. However, these changes alone do not predict cancer. Transformed cell lines that cause progressively growing tumors tend to have fewer alterations in carbohydrates of complex polysaccharides than lines causing tumors that regress.