The method consists in inoculation of double strength liquid enrichment medium in three tubes with a specified test sample, inoculating of a single strength liquid enrichment medium in three tubes with a specified test sample; then under the same conditions, inoculating of single strength medium with decimal dilutions. Incubation of the tubes at 35 C or 37 C for 24 to 48 h, and examination of the tubes for gas formation. After further treatments determination of the MPN by means of an MPN table (see annexes A and B), according to the number of incubated tubes which gave rise to gas formation in the selective medium and in which indole was produce in trypton water.