The casein-splitting ability of each of seven cultures isolated from Kingston cheese has been studied. The cultures are Gram positive non-gelatin-liquefying coccus forms; some appear as chains in young milk culture, others as pairs. The methods of Orla-Jensen and Wasteneys and Borsook were used.It is shown that: (a) Cultures EMB2166 and 168 fail to attack casein; (b) the casein-splitting abilities of cultures EMB1131, 133, 173, 195 and EMB2173 are established and well-defined; (c) the type of proteolysis characteristic of cultures EMB1173 and 195 leads to the formation of large amounts of the simpler degradation products, whereas the proteolytic breakdown characteristic of cultures EMB1131, 133 and EMB2173 is not as complete; (d) culture EMB2173 is distinct in its casein-splitting abilities, as is evident when the results obtained by the two methods are compared; (e) unable to attack casein, cultures EMB2166 and 168 attack the non-protein-nitrogenous components of milk; (d) culture EMB2166 may be capable of protein synthesizing action.