Specifies a method for enumerating bacteria which, at 30 degrees centigrade, form characteristic colonies and which can ferment lactose with the production of gas under the operational conditions described. The method described is applicable to milk and liquid milk products, dried milk, dried sweet whey, dried buttermilk, lactose, acid casein, lactic casein, rennet casein, caseinate, dried acid whey, cheese and processed cheese, butter, frozen milk products, custard, desserts, and cream. Is to be preferred for samples in which large numbers of coliforms are suspected (more than 100 per gram or 10 per millimetre).