Biosynthesis of fucose containing lacto-series glycolipids has been studied in human colonic adenocarcinoma Colo 205 cells. Transfer of fucose in both alpha 1----3 linkage to type 2 chain acceptors and alpha 1----4 linkage to type 1 chain acceptors was demonstrated with a Triton X-100 solubilized membrane fraction. The enzyme was found to be highly active over a broad pH range between 6.0 and 7.5. Kinetics of the transfer reactions were studied and indicated that the enzyme had an apparent Km for GDPfucose of 53 and 49 microM with acceptors nLc4 and Lc4, respectively. The apparent Km values for acceptors Lc4, nLc4, and IV3NeuAcnLc4 were determined to be 42, 18, and 26 microM, respectively. Transfer of fucose to the type 1 chain acceptor Lc4 alone and in the presence of increasing concentrations of the type 2 chain acceptor IV3NeuAcnLc4 or Gb3 suggested that both type 1 and 2 acceptors were alternate acceptors for a single enzyme. This was further established by the finding that IV3NeuAcnLc4 behaved as a competitive inhibitor of fucose transfer with respect to Lc4. Conditions were defined for preparative scale in vitro synthesis of fucosylated products of nLc6 catalyzed by the Colo 205 cell enzyme. Yields of the monofucosyl derivative of 2.5 mg (46%) and 1 mg (17%) of the difucosyl derivative were obtained from 5 mg of original nLc6. The structures of these biosynthetic products were carefully studied by 1H NMR, +FAB-MS, and methylation analysis. These studies revealed extremely high purity products composed of III3FucnLc6 and III3V3Fuc2nLc6. The significance of the nature of these products and enzymatic properties is discussed.