This paper studied on the role of calcium in IAA-induced swelling of protoplasts isolated from hypocotyl in etiolated mung bean (Phaseolus radiatus L.) seedlings. Protoplasts incubated in CaCl2-bearing medium without hormone maintained a constant volume and a consistent intensity of 45Ca2+ radioactivity. To treat with IAA, they began to swell and continually swelled to the maximum volume 30 minutes later (Fig. 2). However, the protoplasts could not swell when IAA was added into the medium without CaCl2 (Fig. 1). It was suggested that Ca2+ may be necessary for IAA to induce protoplast swelling. And also, IAA enabled the protoplasts to swell in less extent with K+, Zn2+, Ba2+ or Mg2+ instead of Ca2+ (Fig. 3). Radioisotope experiments showed that K+ influx increased when K+ replaced Ca2+ (Fig. 4), and water absorption plays a role in the swelling (Fig. 5). 45Ca2+ accumulation in protoplasts treated by IAA was much higher than that of control, and the time course of 45Ca2+ accumulation was similar to that of protoplasts swelling (Fig. 6). 45Ca2+ level and the swelling of protoplasts sharply declined when EGTA, verapamil or LaCl3 was added into the medium (Table 1, 2 and 3). These results indicated that Ca2+ may play an important role in IAA-induced swelling.