Chromosomal DNA from Streptococcus mutans 6715 (serotype g) was cloned into Escherichia coli K-12 by using the cosmid pJC74 cloning vector and a bacteriophage lambda in vitro packaging system. Rabbit antiserum against S. mutans extracellular proteins was used for immunological screening of the clone bank. Twenty-one clones produced weak to strong precipitin bands around the colonies, but only after the lambda c1857 prophage was induced by being heated to lyse the E. coli cells. None of the clones expressed enzyme activity for several known S. mutans extracellular enzymes. One of these clones contained a 45-kilobase recombinant plasmid designated pYA721. An 8.5-kilobase fragment of S. mutans DNA from pYA721 was isolated and recloned into the BamHI restriction site of the plasmid vector pACYC184 to construct pYA726. pYA726 contained all, or nearly all, of the gene for a surface protein antigen (the spaA protein) of S. mutans 6715. This was deduced from immunological studies in which extracts of cells harboring pYA726 reacted with antisera against both purified 6715 spaA protein (about 210,000 daltons) and the immunologically similar antigen I/II of serotype c strains of S. mutans. In addition, the S. mutans spaA protein was found to possess at least one antigenic determinant not present on the protein specified by pYA726. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis of E. coli clone extracts revealed that pYA726 produced a polypeptide with a molecular mass of about 180,000 daltons which was predominantly found in the periplasmic space of E. coli cells. Antisera to the spaA protein of S. mutans reacted with extracellular protein from representative strains of S. mutans serotypes a, c, d, e, f, and g, but not b.