Abstract: Particles having a mean diameter from 0.1 to 50 micrometers and having a solid core of water-impermeable polyacrylonitrile and a partially hydrolyzed hydrophilic surface layer enclosing the core containing from 0.001 to 0.8 milliequivalents of carboxyl or carboxylate groups per dry gram, the surface layer amounting to 0.01 to 20% by weight of the particles, are useful as a solid phase insoluble support to which biospecific reagents such as antigens or antibodies can be coupled. Variants include supports in which N-hydroxysuccinimide or various spacer arms are coupled to the carboxyl or carboxylate groups.