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Hexadecylphosphocholine (HePC) is the first representative of the alkylphosphocholines (APC), a new group of biologically active compounds. HePC has pronounced antiproliferative effects on neoplastic cells in vitro and in vivo. The molecular mechanism by which HePC exerts its biological effects is still under investigation. Recently there has been growing evidence that HePC probably interferes with cellular signalling via phospholipases. It has been shown to inhibit both forms of phospholipase C (PLC), the phosphatidylinositol- and the phosphatidylcholine-specific PLC, and phospholipase A2. Here we present data showing that HePC inhibits the activity of phospholipase D in vitro, whereas the action of this enzyme in leukemic cell lines is not affected. Furthermore HePC does not seem to disturbed the activity of sphingomyelinase, another enzyme of phospholipid metabolism which has been shown to play an important role in cellular signalling as well.