An investigation was undertaken in order to ascertain to what extent and in what manner the individual species and varieties of lower fungi and bacteria present in liquid and solid media affect the simultaneous developmentof other similar organisms. The preliminary experiments were carried out with a culture of streptococcus lactis acidi, which was added in varying proportions to cow's milk immediately after milking, and gelatine plates were made at once, partly from the milk to which the cultures had been added and partly from the natural milk. The gelatine employed contained either glucose and peptone (Witte), or in addition an excess of calcium carbonate. The proportion in which the culture was added to the milk varied from 1 c.c. in 500 c.cs. of milk to one drop of the culture in half a gallon of milk. The results were in all cases practically the same, namely, thatthe plates infected with the normal milk gave an abundant development of bacteria and were in all cases rapidly liquefied, whereas the plates made from the infected milk showed a far less varied bacterial contents and a total suppression of the liquefying species.