A method is presented which allows the direct quantification of respiratory active bacteria in water and biofilm samples. The detn. is performed with a fluorescent redox dye (5-cyano-2,3-ditolyltetrazolium chloride [CTC]). By the fluorescence, a very sensitive detection of reducing activity can be achieved, even on opaque surfaces. The activity of the biomass can be detd. as well as the activity of single cells. The latter can be performed microscopically by detn. of the light yield, because a readily detectable, fluorescent crystal is formed inside the cell by its reductive activity. The method is easy to carry out and takes only 1-2 h. It gives the no. of physiol. active cells and can be combined with other methods, giving the total cell no. The CTC test is suitable for routine measurements as well as for monitoring. It is also useful to assess the effectivity of biocide application.