A 14C trisaccharide synthesised by cultures of Betacoccus arabinosaceous in a medium containing lactose and 14C sucrose has been isolated. This was characterised as the branched structure 0-B-D-galactopyranosyl D-glucopyranosyl D-glucose. The distribution in the trisaccharide was determined by degradation methods. It has been shown that ca. 98% of the activity was located in the non-reducing glucose unit linked to C2 of the reducing moiety of lactose. The acceptor specificity of dextransucrase preparations from B. arabinosaceous have been investigated by incubating certain simple sugars, and related compounds, with the enzyme and sucrose. Chromatography of the digests, and of hydrolysates of the oligosaccharides isolated from them, indicated that D-xylose, D-ribose, L-sorbose, D-mannose, myoinositol, and possibly L-rhamnose, L-fucose, D-arabinose, and methyl B-D-ribopyranoside, acted as acceptors. From consider ation of the conformations of these compounds, and ofthose already established as acceptors in the dextran-sucrase-sucrose system a hypothesis has been put forward as to the essential structure for a glucosyl acceptor. Oligosaccharides synthesised by B. arabinosaceous in a galactose-sucrose medium were isolated and investigated. One of these was characterised as a new nonreducing disaccharide, O-a-D-glucopyranosyl D-galacto-furanoside. Evidence has been obtained that a second disaccharide was 0-a-D-glucopyranosyl D-galactose. Possible tri- and tetrasaccharides were also separated from the culture. Total and partial hydrolysis of these sugars suggested that they were synthesised by successive 1:6 additions to a galactose-containing disaccharide. 14C tracer techniques have been used to investigate the reversibility of the dextransucrase reaction. It has been shown that the enzyme does not catalyse the transfer of glucosyl units from dextran, or from the 'branched' trisaccharide, mentioned above, to other acceptors. It has, however, been demonstrated that, inthe presence of sucrose and 14C fructose, the enzyme catalyses the incorporation of 14C fructose into sucrose. The significance of the transfer of glucosyl units to sugars in the furanose form has been discussed.