SUMMARY:The heat resistance of a number of strains ofStreptococcus lactisandStr. faecalisat 60° in Ringer's solution and separated milk has been determined and the Decimal Reduction Times (D.R.T.) calculated.Heating in separated milk at 60° enabledStr. lactisto be sharply differentiated in most cases fromStr. faecalis.The latter gave D.R.T. values usually much above 3 min. while the former gave values usually under 1 min. and certainly less than 3 min. Single tests might give misleading results with certain strains.The log. survivor‐time graphs of both species were not always straight lines over all their length. A preliminary period of slow death was frequently obser