Forty-five young adults were divided into three groups of equal size and given a professional toothcleaning (3 times during 1 wk) with dental gels containing either chlorhexidine (CHX, 1%), or a SnF2-amine fluoride (AmF) combination (F- content 1.20%). The control group received toothcleaning with a placebo gel. The study was done double blind. The number of salivary mutans streptococci was monitored for 11 wk after the gel treatment using both mitis-salivarius-bacitracin agar plates and the chairside method based on the adhesion of mutans streptococci on plastic strips (Dentocult-SM Strip mutans). Professional toothcleaning with a CHX gel was clearly most effective (P less than 0.001) but the baseline levels of streptococci returned in 11 wk. SnF2-AmF gel also reduced significantly (P less than 0.001) mutans streptococci but recolonization occurred already in 7 wk. Placebo gel, i.e. professional toothcleaning as such, did not show any statistically significant effect on the numbers of salivary mutans streptococci. None of the treatments affected the levels of lactobacilli or the total aerobic flora in saliva samples. Professional toothcleaning with a CHX gel, or with a SnF2-AmF gel, can be regarded as an alternative to the commonly used, but more patience demanding, application of gel in mouth trays at home for individuals with high caries activity.