There is an increasing consumer interest in the microwave oven as a more convenient and quicker means of meal preparation. This study investigated whether growth or inactivation of microorganisms in the microwave field follows the same dynamics as conventional heat processing. Product safety during microwave treatment of food products is of special interest. As parameters D-values of Escherichia coli, Salmonella typhimurium and Staphylococcus aureus were calculated after microwave exposure and conventional heat treatment at +55 degrees C and +60 degrees C respectively. The irradiation frequency was 2450 MHz; the microwave power ranged from 0 to 1037 W. Furthermore an assessment was made on the growth rates of E. coli at +37 degrees C and on the influence of microwaves on lyophilized E. coli-cells. With a special temperature measurement system (Luxtron 1000 A) which used nonmetallic and microwave transparent fiber optic probes, the temperature was recorded during each experiment. At certain temperatures some of the strains showed slight although significant differences depending on which of the abovementioned techniques had been applied. However there was no particular trend evident from the results. D-values of E. coli at +55 degrees C and S. typhimurium at +55 degrees C and +60 degrees C obtained from both heat sources were coincided. Microwave reduction of S. aureus at +55 degrees C was more rapid than conventional heat inactivation; on the other hand a slower inactivation rate of S. aureus and E. coli at +60 degrees C was observed. Growth of E. coli was slightly delayed during microwave incubation. There are no effects concerning microwave-treated E. coli-cells. The hypothesis positing the existence of so called "athermal effects" was neither proved nor rejected on the basis of the experiments. In terms of product safety, it must be taken into account that microwave heat processing in general use may result in a markedly uneven distribution of temperature within the product. Adequate means should be provided for heat conduction so as to allow temperatures of "hot" and "cold" spots to be sufficiently equilibrated.