Micron-size reactive polymeric microspheres with active ester groups on their surfaces were prepared by the seeded copolymerization of a polymerizable surfactant bearing an active ester group, namely p-10-undecenoyloxyphenyldimethylsulfonium methylsulfate (UPDS), and various dialkyl fumarates (DRFs), using 2,2’-azobis(2-amidinopropane)dihydrochloride (AIBA) and 2,2’-azobisisobutyronitrile (AIBN) as the initiators in aqueous systems. The DRFs were incorporated into monodisperse polystyrene (PS) microspheres of 4.1 mu m diameter, and UPDS was then adsorbed on to the surface of the microspheres. When diethyl fumarate (DEF) was used as the comonomer, a bridging flocculation occurred even at a concentration of UPDS below the critical micelle concentration (c.m.c.), because the copolymer of UPDS with DEF is soluble in water. On the other hand, when di-n-butyl fumarate (DBF) or dioctyl fumarate (DOF), which both form water-insoluble copolymers with UPDS, were used as comonomers, and the concentrations of UPDS were lower than its c.m.c., the copolymerization of the adsorbed UPDS with, for example, DBF inside the particles, gave microspheres with active ester groups on their surfaces, without the formation of any new particles. The concentration of the active ester groups on the surface of the microspheres was controlled by keeping the initial particle size constant. The resulting microspheres modified with UPDS units on their surfaces were found to be highly reactive with the primary amine group in an aqueous medium.