The antibacterial action of three different types of propolis extracts: (i) water-extracted propolis (WEP), (ii) propolis volatiles (PV), and (iii) ethanol-extracted propolis (EEP) were investigated by flow microcalorimetry coupled with polarography, and by Petri dish bioassay methods. The water-extracted propolis solution had the weakest antibacterial and antifungal action, compared to the other two extracts, which showed effects nearly similar to each other. Filamentous fungi were generally less sensitive to propolis than bacteria and yeasts, regardless of the type or concentration of propolis. Propolis displayed both bacteriostatic and bactericidal actions depending on the concentration, type of propolis, and type of bacteria tested. The Gram negative bacterium E. coli was insensitive to most treatments, and higher concentrations of propolis were required to achieve bactericidal effects. Treatments of bacteria with weak propolis concentrations caused a decrease in the calorimetric power-time (p-t) curves to lower levels, at which the curves remained for the rest of the experimental period, decreased to the baseline with the course of time, or revived after some time and attained peaks. The treatment with strong concentrations, however, caused the curves to descend to the baseline immediately. (C) 2004 Elsevier B.V. All rights reserved.