A mutant of wild-type Streptococcus sobrinus 6715-13 has been isolated which resists aggregation by exogenous dextran. This variant is able to form adherent plaque deposits in vitro when cultured in the presence of sucrose and has dextranase activity. In these respects it is the complement of previously described isolates which are plaque formation defective but aggregation normal. Measurements of the incorporation of glucose from glucosyl-labeled sucrose into glucan by cell-associated glucosyltransferase enzyme activity and the thermal labilities of catalytic and receptor functions, as well as the binding of labeled dextrans to the cells, provide evidence that neither dextranase nor glucosyltransferase is the receptor involved in dextran-induced aggregation. Blockage of such bacterial aggregation by anti-glucosyltransferase or anti-dextranase sera suggests cross-reactivity between the antigenic determinants of proteins which recognize alpha(1-6) glucan linkages. A model is proposed, consistent with these and previous findings, in which enzymatic function precedes dextran receptor activity in emergence from the cell. It is also proposed that dextran receptor components of the multireactive glucosyltransferase enzyme(s) and dextranase(s) are spatially separate from, although functionally and antigenically related to, the receptors on the bacterial surface involved in dextran-induced aggregation.