The purpose of the present study was to determine by means of a fluorescence test the ratio between vital and dead bacteria in dental plaque before and after 0.1% chlorhexidine rinses. Plaque was stained by fluoresceindiacetate (FDA), which gave a green color to the living micro-organisms, and by ethidiumbromide (EB), which introduced a red color into the nucleic acids of the dead bacteria. 1-, 2- and 3-day-old undisturbed plaque harbored 80 to 85% living micro-organisms. 1 h after chlorhexidine use, this % was reduced to a plateau of 19 to 34% of vital bacteria present in the sampled plaque. Between 6 and 8 h later, this % rose towards its original value. This study has shown that FDA/EB staining provides a method for direct measurement of bacteria vitality in dental plaque and, as such, can be of great value for testing the efficacy of various antiplaque agents.