The technique of microcalorimetry allows observation of the effects of antimicrobial agents on bacterial metabolism without interruption. In contrast to the traditional methods, microcalorimetry assesses the thermal activity rather than the multiplication of bacteria. With one exception - polymyxins show a rapid fall of the power-time-curve to the zeroline, just like disinfectants - one can clearly differentiate bacteriocidal chemotherapeutics from bacteriocidal disinfectants using microcalorimetry. Bacteriocidal chemotherapeutics are not able to put an immediate stop to the metabolic activity of the bacteria.