Summary. Suspensions of the cariogenic bacteria Streptococcus mutans, S. sobrinus, Lactobacillus casei and Actinomyces viscosus were exposed to light from a 7·3-mW heliumneon laser in the presence of toluidine blue O. A substantial killing rate (c. 106 cfu) of all four species was achieved with a dye concentration of 50 μg/ml and a light energy dose of 33·6 J/cm2. This was achieved in 60 s, an exposure time that is clinically acceptable. Exposure to laser light in the absence of the dye did not significantly affect the viability of any of the organisms. This approach may be useful in dentistry to sterilise a carious lesion prior to its repair.