A carrier for a biologically active component for immunoassay or enzymatic reaction, which comprises:a) a thermoplastic resin bead having an average diameter of from 0.05 to 20 mm,b) from 1 to 25% by weight, based on the bead, of a magnetically responsive powder bonded to the bead, andc) a polymer coated thereon in a thickness of from 2 to 30 .mu.m, said polymer having a number average molecular weight of from 200 to 10,000 and having functional groups capable of binding, or being activated to bind, the biologically active component.