Lysophosphatidylcholine, arachidonic acid and docosahexaenoic acid were found to have a concentration-dependent inhibitory effect on the motility of human sperm, whilst phosphatidylcholine had no effect. Seminal plasma attenuated the sperm-immobilizing potencies of these lipids. Because all of the three inhibitors of motility are hydrolytic products of phosphatidylcholine, and the catalytic enzyme, phospholipase A2, is known to be calcium dependent, it is suggested that calcium might inhibit sperm motility by activating phospholipase A2 which in turn releases lysophosphatidylcholine and free fatty acids.