ALTHOUGH different prostaglandins* may have different pharmacological effects, in general the compounds have the same qualitative action on a given cell type. Investigation1,2 has revealed structure-activity correlations and the importance of the orientation of the 11 and 15 hydroxyl groups3 is recognized. Molecular model building together with structure-activity data have suggested that prostaglandins are one of a number of classes of drugs and hormones that seem to fit stereochemically into a pre-existing (for example, polypeptide-nucleotide) receptor site4,5. The restraints applied to the prostaglandin molecule to fix it as a stable conformational isomer, or conformer, are little understood and we now wish to indicate the significance of this conformation with respect to interaction with biological membranes.