A cell-free enzyme solution prepared from cultured cells of Phaseolus vulgaris mediated C-24 methylation of 28-nor­castasterone to castasterone with the aid of S-adenosylmethionine as a co-substrate in the presence of the NADPH cofactor. This enzyme solution also catalyzed conversion of 28-norcastasterone to a de methylated 28-norcastasterone, most likely 26,28-didemethyl-castasterone, when S-adenosylmethionine was not added to the enzyme solution. Furthermore, gene expression of Arabidopsis CYP85A1 and CYP85A2 mediating the conversion of 6-deoxo-28-norcastasterone to 28-norcastasterone was strongly inhibited by treatment of 28-norcastasterone. These results suggest that 28­norcastasterone, along with castasterone and brassinolide, is an important brassinosteroid whose endogenous level should be strictly controlled to express brassinosteroid activities in plants.