The synthesis of α (?)-galactose-l-phosphoric acid has provided a means of examining the constitution of the galactosephosphate accumulating in the liver during galactose assimilation1. Both natural and synthetic esters are non-reducing, very readily hydrolysed by acid, and resistant to alkaline hydrolysis. After removal of the reducing esters by alkaline hydrolysis, the mixture of phosphoric esters isolated from the liver during galactose assimilation contained, apart from water of hydration, 29·6 per cent anhydrous barium galactose-l-phosphate, 3·1 per cent anhydrous glucose-l-phosphate, 2 per cent barium salt of an unidentified acid-labile ester, and 60 per cent barium salt of an acid-resistant ester. This mixture of barium salts had [α]5461 + 34·4° and the barium salt of the acid-resistant ester [α]5461 – 3·4°. Cori, Colowick and Cori2 found for anhydrous barium glucose-l-phosphate [α]D + 75° which corresponds to [α]5461 + 89°. By calculation from these values, natural barium galactose-l-phosphate would have [α]5461 + 114° while the corresponding value of the synthetic ester is + 110°. Determined in a similar manner, the constant of hydrolysis of the natural ester in 0·25 N HCl at 25° would be 0·87 × 10-3 while that of the synthetic ester is 0·89 × 10-3. These findings, together with the earlier evidence, make it highly probable that the galactosephosphate present in the liver during galactose assimilation is identical with synthetic α(?)-galactose-l-phosphoric acid. An attempt is being made to crystallize the natural ester as the dipotassium salt.