When 1-methacryloxybenzotriazole (MABt) and acrylonitrile (AN) were polymerized in ethyl propionate by γ-irradiation, the homogeneous solution changed into an unstable emulsion during irradiation, resulting in formation of polymeric microspheres, in which the particle diameter of homopoly(MABt) was ca 2 μm, in contrast to 0.4 μm for homopoly(AN), under such irradiation conditions as 30 kGy, 25 and 10 w/v% monomer concentration. The particle size of copolymeric microspheres can be controlled in the range of 2-0.4 μm, according to monomer composition. The reactivity of copoly(MABt/AN) microspheres with active Bt group was evaluated from the amount of 1-hydroxybenzotriazole (HOBt) liberated by reacting with alkylene diamine derivatives, represented by a general formula H 2N(CH 2) n NH 2 ( n = 2, 3, 5, 7 and 9). As a result, when a mixture of 1 g of copoly(MABt/AN, 80/20 wt%) microspheres, 3.0 ml of ethylene diamine ( n = 2), and 50 ml of 0.1 M phosphate buffer solution (pH 8.0) was reacted for 1 hr at 40°, the amount of liberated NOBt was found to be 410 mg/g. In this case, the reactivity showed a linear decrease with increasing number of methylene units in H 2N(CH 2) n NH 2.