The question of physiol. activity of microorganisms is of great interest in many microbially caused problems. 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 means of fluorescence a very sensitive detection of reducing activity can be achieved, even on non-transparent surfaces. Both biomass activity and the activity of single cells can be detd. The latter can be obtained microscopically by measuring the light yield, because a readily detectable, fluorescent crystal is formed inside the cell after redn. has occurred. 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, e.g., detn. of 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.