(–)-L-threo-PDMP is a ceramide analog and one of the four possible stereoisomers of PDMP . L-threo-PDMP is an enhancer of ganglioside biosynthesis. It increases glucosylceramide synthase activity by 4 and 8% when used at concentrations of 5 and 2.5 µM, respectively, in an enzyme assay. L-threo-PDMP increases the activity of ganglioside GM, GD, and GQ synthases and increases the levels of glucosylceramide, lactosylceramide, and ganglioside GM in B16 melanoma cells. It inhibits the growth of MCF-7, MDA-MB-468, SK-BR-3 cells with IC values of 4, 7, and 6 µM, respectively. L-threo-PDMP increases neurite outgrowth and synapse formation in vitro and improves retention of a long-term memory learned prior to forebrain ischemia in rats when administered following ischemia at a dose of 40 mg/kg per day. The L-threo-PDMP isomer is the glucosylceramide synthetase stimulatory component of DL-threo-PDMP , while D-threo-PDMP is the inhibitory component.