Summary: To establish whether the bifidum flora in the bowel may be influenced by oral administration of lactose, primarily beta lactose, the following investigation was conducted. A number of healthy children between the ages of 2 weeks and 2 ½ years were given alpha and beta lactose for separate periods of about 15 days (6—20 days). During these periods regular tests were made of the stools, with respect to physical characteristics, pH and bifidum count. In addition the number of bacteria growing on endoagar and dextrose agar was determined. The lactose was either substituted, in the same or double the amount, for the usual sucrose in the cow's milk formula, or supplemented the ordinary diet in doses of 3.5 g per kilogramme body weight per day. In each case the lactose was dissolved in a liquid immediately before the meal.The investigation tends to show that both alpha and beta lactose, administered by mouth, may produce in certain cases a moderate increase of the bifidum count in the faeces. The increase is not significant. No difference was found between alpha and beta lactose with respect to bifidum‐stimulating capacity. Nor did any unequivocal changes occur in certain other faecal flora or in the physical characteristics of the stools during the period in which lactose was administered.