LIVE/DEAD(R) BacLight(TM) and alamarBlue(R) are fluorescent materials used for the enumeration of live and dead bacteria. LIVE/DEAD(R) BacLight(TM) is generally used for confocal microscopy applications to differentiate live from dead bacteria in a biofilm or planktonic state. AlamarBlue(R) has also been used widely to assay live and dead bacteria in a planktonic state. Whilst these materials are successfully utilized in experiments to discriminate live from dead bacteria for several species of bacteria, the application of these techniques to oral bacteria is limited to the use of LIVE/DEAD(R) BacLight(TM) in biofilm studies. In our present study, we assessed whether these two methods could enumerate live and dead oral bacterial species in a planktonic state. We tested the reagents on Streptococcus mutans, Streptococcus sobrinus, Porphyromonas gingivalis, Aggregatibacter actinomycetemcomitans and Enterococcus faecalis and found that only LIVE/DEAD(R) BacLight(TM) could differentiate live from dead cells for all five of these oral strains. AlamarBlue(R) was not effective in this regard for P. gingivalis or A. actinomycetemcomitans. In addition, the differentiation of live and dead bacterial cells by alamarBlue(R) could not be performed for concentrations lower than 2 x 10(6) cells/ml. Our data thus indicate that LIVE/DEAD(R) BacLight(TM) is a more effective reagent for this analysis.