PURPOSE: To obtain the titled fine particles suitable for a carrier capable of immobilizing a biologically active material easily with high reproducibility, by reacting hydrogen sulfide, etc. with fine copolymer particles consisting of glycidyl acrylate to introduce mercapto groups thereinto.

CONSTITUTION: (B) Hydrogen sulfide, a polythiol having two or more mercapto groups, e.g. dithioglycol, or an aminomercaptan having mercato groups and amino groups together, e.g. 2-aminoethanethiol, is reacted with (A) fine particles, consisting of a polymer having repeating units of glycidyl acrylate and/or glycidyl methacrylate, and having 0.03W20μm average particle diameter to introduce mercapto groups thereinto and give the aimed fine particles. A water- soluble monomer is preferably used as another copolymerization component in the component (A) to give the following advantage; Nonspecific aggregation is scarcely caused for a specimen humor, etc.