1. Maltase sucrase, palatinase (the enzyme that hydrolyses palatinose, i.e. 6-o-α-D-gluco-pyranosyl-D-fructose) and lactase activities were measured in the small and large intestines of germ-free and conventional chicks given either a diet of purified ingredients or a practical chick mash.

2. With the purified diet there were no differences in body-weight or small intestinal disaccharidase activities between germ-free and conventional chicks. With the chick mash the germ-free birds were heavier and had higher total amounts of maltase, sucrase and palatinase activities in the small intestine than did their conventional controls. When disaccharidase activities were expressed in terms of body-weight there were no differences between birds in the two environments. Enzyme activities were consistently higher in the birds given chick mash.

3. Inclusion of milled fibre in the purified diet did not increase the weight or disaccharidase activities of the small intestine in either environment.

4. Lactase was virtually absent from the small intestine of birds in both environments and from the large intestine of germ-free birds. There was appreciable lactase activity in the large intestinal contents of conventional chicks, and it was increased by inclusion of lactose in the diet.

5. When lactose was the sole source of carbohydrate the birds grew poorly but mortality rate was less among conventional compared with germ-free chicks.

6. It was concluded that the presence of micro-organisms has no direct effect on disaccharidase production in the small intestine of the chick. Microbial lactase is present in the large intestine, and at least some of the products of its action can be utilized by the bird.