Water-insoluble, non-adherent alpha-D-glucans have been obtained from Streptococcus salivarius HHT under two sets of conditions: from a growing culture, or synthesized enzymically by using a glucosyltransferase. In the former case, the glucan ([alpha]D + 197 degrees) was shown by methylation analysis to have a slightly branched structure containing a relatively high proportion (80%) of (1 leads to 3)-alpha-D-glucosidic linkages, together with small proportions of (1 leads to 6)- and (1 leads to 4)-alpha-D-glucosidic linkages. The enzymically synthesized glucan had a much less-branched structure, containing 88% of (1 leads to 3)-alpha-D-glucosidic linkages. Both glucans, on Smith degradation (sequential periodate oxidation, borohydride reduction, and mild acid hydrolysis), gave linear, (1 leads to 3)-alpha-D-glucosidic polysaccharides (yields, 82--90%) that constitute the backbone chains. The presence of small proportions of glycerol, erythritol, 1-O-alpha-D-glucosyl-D-glycerol, and also 2-O-alpha-D-glucosyl-D-erythritol in the products of Smith degradation suggests that the short side-chains are attached to the backbone chain by (1 leads to 4)-, (1 leads to 6)-, and (1 leads to 3)-alpha-D-glucosidic linkages.