– C6 L-threo Ceramide is a bioactive sphingolipid and cell-permeable analog of naturally occurring ceramides. It is cytotoxic to U937 cells in vitro (IC50 = 18 μM), metabolically inactive, and unlike C6 L-erythro ceramide, it cannot be converted to C6 glucosylceramide by ceramide glucosyltransferase. C6 L-threo Ceramide enhances IL-4 production induced by phorbol 12-myristate 13-acetate in EL4 T cells at 10 μM.