.alpha.-Galactosylceramides and glycosylceramides ("ceramide-like glycolipids") that modulate NK T cells. The ceramide-like glycolipids vary in the cytokines induced in NK T cells and vary in the antigen-presenting cells that are capable of efficiently presenting the compounds to NK T cells. Pharmaceutical compositions of the ceramide-like glycolipids are provided, as are pharmaceutical compositions of the ceramide-like glycolipids combined with dendritic cells. Methods utilizing the ceramide-like glycolipids in vaccines, to activate NK T cells, to stimulate the immune system, and to treat mammals are also provided. The invention also provides methods of evaluating a compound for its ability to activate an NK T cell in the presence of a cell expressing a CD1d protein.