Highly uniform polymeric microbeads of a predetermined size, preferably having an attached fluorescent label on the surface, are prepared by causing seed particles to swell by absorption of a plurality of low molecular weight substances, at least one of which is a monomer having a reactive functional group. After absorption, the monomer is internally polymerized, and the reactive functional group is preferably reacted with a polyfunctional spacing agent thereby to provide a second reactive functional group. The latter is then reacted with a fluorescent label to provide the labeled microbead.