This paper studied on the role of calcium in 6-BA-induced swelling of protoplasts isolated from hypocotyl in etiolated mung bean(Phaseolus radiatus L.) seedlig. Protoplasts incubated in CaCl2-bearing medium without hormone maintained a constant volume. To treat with 6-BA, they began to swell after 15 minutes, and continually swelled to the maximum volume 30 minutes later when they were treated with 10-1 mol/L 6-BA(Fig.2). However, the protoplasts could not swell when 6-BA was added into the medium without CaCl2(Fig.1). It was suggested that Ca2+ was necessary for 6-BA to induce protoplast swelling. 6-BA enabled the protoplasts to swell in less extent when K-. Zn2+, Ba2+or Mg2-instedad of Ca2-(Fig.3). Radioisotope experiments showed that K+influx incrased when Ca2+ as replaced by K+(Fig.4). Ca2+ accumulation in protoplasis treated by 6-BA was munch higher than that of control, and the time course of Ca2+ accumu-lation was similar to protoplasts swelling (Fig5). 45Ca2+level and the swelling of protoplasis were sharply declined when EGTA, verapamil or LaCcl3 was added into the medium(Table 1,2and3). These results indiated that Ca2+ may play an important role in 6-BA-induced swelling.