para-Nitrophenyl-(alpha)-D-glucopyranoside was found to be a substrate for the glucosyltransferases produced by several species of Streptococcus and Leuconostoc bacteria. It was found to react to produce both high molecular weight dextran and acceptor products with the glucosyltransferase of Leuconostoc mesenteroide B512F;The glucosyltransferases were also found to react with alternate glucosyl donors such as dextran, maltotriose, panose, and isomaltodextrins containing three or more glucose units. These reactions seemed to be the reverse of acceptor reactions;In order to study the sucrose binding sites of these enzymes, techniques were developed for the synthesis of sucrose analogs. By these techniques, we synthesized 6-deoxysucrose, 6-thiosucrose, 3-deoxysucrose, 3-deoxy-3-fluorosucrose, and allosucrose;The sucrose analogs were tested as substrates and/or inhibitors of the glucosyltransferases synthesized by Streptococcus mutans 6715, GTF-I which synthesizes a water insoluble glucan, and GTF-S which synthesizes a water soluble glucan. 6-Thiosucrose, 3-deoxysucrose, and 3-deoxy-3-fluorosucrose were all glycosyl donors for both enzymes as judged by the formation of acceptor products with maltose. All of the analogs were inhibitors to some extent for the enzymes. The best inhibitor was 6-deoxysucrose with a competitive K(,I) one order of magnitude lower than the K(,m) for sucrose.