A further investigation has been made of the pathway of the photo-formation of lumisterol, which had remained doubtful after the previous analyses of irradiation mixtures, in which lumisterol was determined indirectly3. Experiments using tachysterol3-7-3H reveal that lumisterol is not formed directly from tachysterol to a significant extent. Thin layer chromatograms of mixtures obtained after various times of irradiation starting with solutions of pre-calciferol3 and tachysterol3 indicate the direct formation of lumisterol3 from precalciferol3. The modified scheme of interrelationships between pro- and pre-vitamin D, lumisterol and tachysterol is essentially the one originally postulated by Velluz, Amiard and Goffinet11. The question whether ergosterol and tachysterol yield other photo-products besides precalciferol remains to be investigated.