A method is described to estimate the microbial biomass of a sample, to enumerate the cells, and to distinguish the portion of metabolically active cells in the population by measuring the binding of phenyldicarbaundecaborane (PCB-) to the cells. This method can also be used for the analysis of a complex population of microorganisms if the cells composing the sample are sensitive to different biocidal agents. In addition, the analysis of PCB- binding is useful for the enumeration of the phage-infected cells and phage particles.