Microsomes isolated from corn embryos (Zea mays) were shown to catalyse the C-4 monodemethylation of 28-[3H],24-methylene cycloartanol 1, leading to the corresponding 4 alpha-methyl sterol, cycloeucalenol 5. An enzymatic assay has been developed for the 4,4-dimethyl sterol 4-demethylase in higher