A latex partice comprising a vinyl aromatic monomer and an vinyl acrylate ester monomer which is modified by a protein binding modifying monomer to facilitate the binding of a protein via coupling techniques. These latex particles are useful in diagnostic test applications. The vinyl acrylate ester monomer and a vinyl aromatic monomer are dispersed in a ratio between 1:3 to 3:1 of vinyl acrylate ester monomer to vinyl aromatic monomer in an aqueous phase containing a protein binding modifying monomer. The protein modifying monomer can be a vinyl carboxylic acid, such as a acrylic acid in a concentration of from about 0.5 percent to about 10 percent of the monomers. The dispersion is subjected to emulsion polymerization to provide a latex particle in a size range from 0.1 to 1 micron. The latex particle is sensitized with an antibody or antigen via coupling techniques. The resulting sensitized latex particles combine attributes of each monomer to provide utility in difficult protein systems to form latex antigen/antibody complexes. The products of the invention may be used in a variety of diagnostic applications.