With a view to attaining more precise information on the biological activity of 2,3,5-tri-epi-brassinolide analogs, 2,3,5-tri-epi-brassinolide was prepared from 2,3-di-epi-brassinolide by direct epimerization at C-5. Biological activity of 2,3,5-tri-epi-brassinolide in the rice lamina inclination test was nil even at 1 mu g/plant by the single application technique, while with coapplication of indole-3-acetic acid, the activity was ca 1/1000th that of brassinolide, reconfirming that the A/B trans-fused ring junction of brassinosteroids is an essential structural factor for high biological activity. (C) 1999 Elsevier Science Ltd. All rights reserved.