A number of naturally occurring phospholipids, eg the acetal phosphatidic acid derivatives that comprise Darmstoff (1) and the phosphatidylcholine derivative platelet activating factor (PAF), cause contraction of certain visceral smooth muscles and cause platelet activaticn. Because the Darmstoff phosphatidic acids and PAF are structurally similar, it was of interest to compare the biological actions of choline esters of Darmstoff with those of PAF and of ...