IT has been demonstrated previously by several workers1–5 that rat liver microsomes readily convert L-gulonolactone into L-ascorbic acid in the presence of oxygen. Recently, it has been found that the microsomes from the livers of goat and the rat can also convert L-gulonolactone into L-ascorbic acid under anaerobic conditions in the presence of a suitable electron acceptor. Various dyes were tested for their ability to act as electron acceptor with this enzyme; these included methylene blue, phenazine methosulphate, neotetrazolium chloride, 2,6-dichlorophenol indophenol and cytochrome c. Of these only phenazine methosulphate and 2,6-dichlorophenol indophenol were reduced by the enzyme in the presence of a substrate; the ability of 2,6-dichlorophenol indophenol as electron acceptor was about one-fifth compared with that of phenazine methosulphate (Table 1). D-Glucuronolactone has also been found to be converted into L-ascorbic acid (as estimated by Roe and Kuether's6 method after the removal of cyanide by acidification with hydrochloric acid and evaporating off the hydrocyanic acid) by goat liver microsomes under anaerobic conditions in the presence of phenazine methosulphate and 0.05 M potassium cyanide.