Particulate embolotherapeutic compositions are described in which the particles comprise a matrix polymer expressing at surfaces available for interaction with biological molecules or, usually, blood cells, express zwitterionic groups, usually phosphoryl choline groups. The particles have suitable size and compressibility, for instance controlled by controlling the water content, for use in embolotherapy, by administration using catheters or hypodermic needles. The matrix polymer may be crosslinked. Preferably the particles are spherical. Examples are crosslinked polyvinylalcohol matrices coated with a crosslinkable copolymer formed of 2-methacryloyloxyethyl-2′-trimethylammoniumethyl phosphate inner salt with nonionic and crosslinkable comonomers. The coating is stabilised by crosslinking after coating from an organic solvent solution.