Four organisms commonly found in contaminated milk (B. coli, B. subtilis, a staphylococcus, and B. faecalis alkaligenes) were grown in milk in association with lactic streptococci, and the acidity produced in a given time was determined. B. subtilis appeared to stimulate the production of acid. B. coli caused a decrease in acid production in some experiments, and an increase in others. The staphylococcus had a slight stimulative effect. B. faecalis alkaligenes had only a slight effect which was barely beyond experimental error. There was no indication that any of the organisms are responsible for the cessation of acidity production which sometimes occurs in milk during cheese manufacture. However, the results were not conclusive in this connection, since the experimental conditions did not simulate those obtaining in a cheese vat.