In order to examine the biological role of the A/B-ring functional groups of plant-growth-promoting brassinosteroids, twenty-three brassinosteroids with some modifications at rings A and B (1-15 and 22-29) were bioassayed by means of the rice-lamina inclination test. The results showed that 1) removal of one or two hydroxyl groups from the A-ring reduced the biological activity of the steroids; 2) the 7-oxalactone brassinosteroids were almost as active as the corresponding 6-oxo steroids and they were much more active than their regioisomeric 6-oxalactone counterparts; 3) introduction of a double bond at the C-7 position and a hydroxyl group at the C-5 position of 6-oxo brassinosteroids significantly decreased the biological activity of the hormonal steroids. These data suggest that the presence of a 2α, 3α-diol, 7-oxalactone or 6-oxo group, and the A/B-trans ring junction are important for high biological activity.