A study of a large number of wheels of Swiss cheese manufactured at a plant under factory conditions, in an effort to determine what acidity of pure culture starters and what rate of lactic acid fermentation in the cheese in the press were most desirable, led to the following conclusions:1.With the milk used, a Lactobacillus helvetious milk starter with an acidity of 1.0 to 1.09 was most effective. This culture was grown at 37.5–39° C. for 12 hours.2.A 12-hour, 37° C. milk starter of Str. thermophilus should have an acidity of about 0.70 to 0.75 per cent; and a whey starter should have an acidity of 0.30 to 0.33 per cent.3.The pH of the cheese in the press, which is indicative of the activity of the starters, should be about 6.0 to 6.09 after three hours in cheese made from milk with a pH of 6.5–6.6.4.The pH of the cheese in the press should be less than 5.2 after 21 hours and preferably should be between 5.15 and 5.0.5.With good milk a slow development of acidity throughout the first 21 hours usually makes a good cheese. The pH of such cheeses may be as high as 6.1 to 6.25 after three hours in the press, and over 5.2 after 21 hours.6.With a supply of good milk and with good manufacturing methods, the proper preparation and use of pure culture starters will give Swiss cheese of good average quality.