C6 D-threo Ceramide is a bioactive sphingolipid and cell-permeable analog of naturally occurring ceramides., C6 D-threo Ceramide is cytotoxic to U937 cells in vitro (IC50 = 18 μM). It is metabolically inactive and, unlike C6 L-erythro ceramide , C6 D-threo ceramide cannot be converted to C6 glucosylceramide by ceramide glucosyltransferase. C6 D-threo Ceramide promotes survival of isolated rat spinal neurons when used at concentrations up to 2.5 μM but induces cell death at concentrations greater than 5 μM. It enhances IL-4 production induced by phorbol 12-myristate 13-acetate in EL4 T cells when used at a concentration of 10 μM.