It was reported previously that pretreatment of rats with a crude preparation of stem bromelain, a thiolprotease obtained from pineapple plant, effectively protected the animals against adrenaline-induced pulmonary edema (1-3). The present study was undertaken to examine whether the edema-preventive effect of the enzyme is due to its proteolytic activity. For this purpose, purified and activated stem bromelain was prepared. A part of this was then inactivated by N-ethylmaleimide, a specific SH-inhibitor. The edema preventive effects of the two enzyme preparations were compared. © 1968, The Japanese Pharmacological Society. All rights reserved.