Several new analogues of L-fucose modified in the 2 position and the 5-methyl group have been synthesized as potential plasma-membrane glycoconjugate inhibitors or modifiers, and their biological effects have been studied. 2-Chloro-, 2-bromo-, and 2-iodo-2-deoxy-L-fucose (9a, 9b, and 13, respectively) have been prepared by addition of the appropriate halogen to 3,4-di-O-acetyl-L-fucal, followed by hydrolysis of the anomeric halogen and the acetyl groups. A series of four halogenated 5-methyl analogues of L-fucose (4, X = F, Cl, Br, and I) have been obtained starting from 1,2:3,4-di-O-isopropylidene-L-galactose. The synthesis of this latter compound has been improved. A corresponding series of 6-deoxy-6-halo-D-galactose analogues, which are enantiomers of the 5-(halomethyl)-L-fucose analogues, has also been synthesized. Analogues 4b, 4c, and 9b at 1 x 10(-3) M specifically inhibited the incorporation of L-[3H]fucose into macromolecular components of SW613 human mammary tumor cells. Analogue 13 inhibited the growth of L1210 murine leukemic cells with an IC50 of 6 x 10(5) M in culture. 6-Deoxy-6-fluoro-D-galactose and its enantiomer 4a were found to be effective inhibitors of D-[3H]galactose and L-[3H]fucose incorporation, respectively, into macromolecular components of human mammary tumor cells. The effectiveness of inhibition was reduced with an increase in size of the halogen atom. Analogue 4a and its enantiomer have been tritiated at C-1 and both were found to be activated to a nucleotide sugar, which was followed by incorporation into the macromolecular fraction of SW613 human mammary tumor cells in vitro.