We have shown that the ABC transporter, multiple drug resistance protein 1 (MDR1, P-glycoprotein) translocates glucosyl ceramide from the cytosolic to the luminal Golgi surface for neutral, but not acidic, gly-cosphingolipid (GSL) synthesis. Here we show that the MDR1 inhibitor, cyclosporin A (CsA) can deplete Gaucher lymphoid cell lines of accumu-lated glucosyl ceramide and Fabry cell lines of globotriaosyl ceramide (Gb3), by preventing de novosynthesis.