Streptococcus mutans has been identified as the principal etiological agent in human dental caries. Therefore, the specificity and sensitivity of 3 potential gene probes derived from S. mutans GS5 have been examined as potential reagents for detecting the cariogenic bacteria. The gene probes derived from the cloned gtfB, gtfD and ftf genes were examined in Southern blots with a panel of representative oral bacteria. The gtfB and ftf genes were apparently specific for S. mutans under high-stringency hybridization conditions. However, the gtfD gene fragment did cross-hybridize with the DNA from other mutans streptococci. These results suggest that gtfB and ftf gene fragments may be used as specific probes for S. mutans.