An active starter is generally considered one of the prerequisites for making a high quality cheese. Slow starters have caused difficulties in commercial plants and several theories as to what may be the cause of the slowing u p of the activity of starters have been formulated. The inability of certain strains of streptococci to grow at cheese scalding temperature appears to be a cause of slowness in cheese making (7, 8, 17). The presence of a bacteriophage also has been given as a cause for slow starters (1, 9, 19). Multiple strains of streptococci are less susceptible to attack by phage than when only one or two strains are present in the culture (1, 9, 19). Slow starters develop sooner with light inoculations, than when heavy inoculation is practiced (1, 6, 20). The phage is introduced by air contamination (21) and has been found associated with equipment, walls and floor which have come in contact with whey (9). Aeration of the milk is a possible source of slow acid development (6, 17, 20). Highest quality milk of high total solids is desirable for starters (2, 4, 10). The presence of certain non-acid streptococci is given as a cause of slow acid production (3, 16). Mastitis milk will slow up the development of acid by the starter organisms (5, ]4). Proper pasteurization of the milk will partially overcome this retarding effect in some samples (12). The exact procedure to which milk is subjected during pasteurization and cooling prior to incubation is of prime importance (6, 15). Milk of late lactation is suggested as one of the causes of slow starters (6). Constant overripening of the starter is suggested as a cause of slow acid development (4). However, Dolby (7) has shown that, given the right treatment, there should be no difficulty in producing cheese of normal acidity using starters of widely varying activity. Whitehead and Cox (18) state that the organisms selected for active starters must have two properties ; (a) They must be active acid formers at temperatures of 20 ° to 30 ° (b) They must be relatively unaffected in their own growth and morphology at 37 ° C. Whitehead and Cox (13) describe a method for determining the relative activity of starters. The principle of the test is to simulate the cheese making process and compare