Abstract: The present invention relates to materials which comprise a surface having pendant groups of formula (I), in which the groups --B-- are the same or different, preferably the same, and each is --CH.sub.2 -- or --C(.dbd.O)--, preferably --CH.sub.2 --, R.sup.1 is hydrogen alkyl of 1 to 12, preferably 1 to 4 carbon atoms, or a group capable of bonding to, or bonded to, a ligand or R.sup.1 is a polymerisable, preferably ethylenically unsaturated, group and Z is a zwitterionic group. Usually both groups B are the same and are joined to identical groups so that the compound of formula (I) is achiral. New compounds are achiral glyceryl phosphatidyl choline analogues. Surfaces having the groups (I) at their surface have improved biocompatibility, and show reduced fibrinogen absorption, reduced platelet activation, reduced microorganism adhesion and are useful in the production of implants or prostheses, for extra-corporeal circuitry for body fluids, for treating membranes of various type, for contact lenses.