Twenty-five Listeria monocytogenes strains of human and food origin were studied, using low-frequency-cleavage restriction endonucleases and pulsed field gel electrophoresis. Sixteen different restriction patterns were obtained using ApaI and SmaI. The human isolates showed profiles different from those of the food isolates, thus strongly suggesting that the two human infections selected for this study did not originate from the contaminated foodstuffs examined at the same time. This method allowed it to clearly distinguish between strains apparently similar [i.e. belonging to the same serovar and same origin (cheese samples from the same dairy plant)].