α-GalCer is an immunostimulating glycolipid that binds to CD1d molecules and activates invariant natural killer T (iNKT) cells. Here we report a scaled-up synthesis of α-GalCer analogues with modifications in the acyl side chain and/or at the galactose 6′′-position, together with their evaluation in vitro and in vivo. Analogues containing 11-phenylundecanoyl acyl side chains with aromatic substitutions (14, 16–21) and Gal-6′′-phenylacetamide-substituted α-GalCer analogues bearing p-nitro- (32), p-tert-butyl (34), or o-, m-, or p-methyl groups (40–42) displayed higher IFN-γ/IL-4 secretion ratios than α-GalCer in vitro. In mice, compound 16, with an 11-(3,4-difluorophenyl)undecanoyl acyl chain, induced significant proliferation of NK and DC cells, which should be beneficial in killing tumors and priming the immune response. These new glycolipids might prove useful as adjuvants or anticancer agents.