A series of d-galactose derivatives substituted at C-1 and C-6 were tested for active accumulation by everted segments of hamster and rat intestine. 2. d-Galactose and 6-deoxy-6-fluoro-d-galactose were accumulated far more rapidly than 6-deoxy- and 6-chloro-6-deoxy-d-galactose, and this is interpreted as due to hydrogen-bonding at C-6 during the transport process. 3. 6-Bromo-6-deoxy- and 6-deoxy-6-iodo-d-galactose were not actively transported, indicating that the allowed size of substituent at C-6 lies between that of chlorine and bromine atoms. 4. Similar results were obtained at C-1. Both methyl alpha-d-galactopyranoside and methyl beta-d-galactopyranoside were well transported, but methyl beta-d-thiogalactopyranoside and 1-deoxy-d-galactose were not transported; d-galactopyranosyl fluoride was transported, but only poorly. Again hydrogen-bonding is suggested. 5. It is proposed that d-glucose is the ideal structure for active transport and that binding occurs at C-1, C-2, C-3, C-4 and C-6. Loss of two or more of these bonds usually causes loss of active transport. 6. By plotting Lineweaver-Burk plots of the rates of transport of the galactose derivatives, the apparent V and K(m) values were obtained. With hamster intestine both these values were very reproducible. Contrary to expectation, V varied for different sugars. 7. The K(i) of some of the analogues modified at C-1 and C-6 was determined with methyl alpha-d-glucoside as substrate. 8. An attempt to alkylate the carrier by using methyl 3,4-anhydro-alpha-d-galactoside was unsuccessful. There was no evidence that this compound was bound to the carrier.
