To find the best temperature for storing human breast milk in the first test series, bacterial growth was determined and recorded at refrigerator temperatures of 8 to 10 degrees C and at 4 degrees C respectively. During a 3-day observation period, there was a slight decrease in bacterial count (b.c.) at higher temperature whereas the b.c. decrease at the lower temperature was more significant. In another test 72 samples of milk were kept for one month at a temperature of -20 degrees C; the b.c. observed after thawing was substantially smaller than that recorded before freezing. Pasteurized milk samples were found sterile with the method we used (in 0.1 ml). In a third test a measured quantity of specific bacteria was added to the milk samples which were first deep-frozen, thereafter pasteurized and finally stored in a refrigerator for 3 days at 4 degrees C. During this process, a decrease was observed in all the bacteria involved. In our opinion, breast milk is best stored at a constant temperature of 4 degrees C. Pasteurization and freezing are also recommended from a bacteriological point of view.