Abstract: Hydrophilic latex particles consisting of a homo- or co-polymer of monomers which are sparingly soluble in water, which hydrophilic latex particles can be prepared by emulsion polymerization in the presence of a water-soluble, radical-forming initiator but without an addition of an emulsifier, stabilizer or wetting agent. A process for the preparation of these hydrophilic latex particles, wherein a monomer which is sparingly soluble in water or several monomers which are sparingly soluble in water are dispersed in water and, with the exclusion of oxygen, for example in an inert atmosphere, are homo- or co-polymerized by emulsion polymerization in the presence of a water-soluble, radical-forming initiator but without any addition of an emulsifier, stabilizer or wetting agent.A diagnostic agent containing these hydrophilic latex particles as carrier and biologically and/or immunologically active substances covalently bound to this carrier either directly or via a coupling agent as a bridge.