and converted into (25S)-26-hydroxycholesterol (6a). The absolute configuration at C-25 of (6a) was determined by X-ray crystallography. (25R)-[26-2H1] Cholesterol (10) was prepared by reduction of the 26-tosyloxy group by LiAl2H4. Reverse-phase h.p.l.c. resolution without derivatiration was developed for the diastereoisomers, (25R)- and (25S)-26-hydroxycholesterol. The (+)- or (–)-MTPA esters of these