1. Germ-free and conventional chicks were given diets containing raw or heated navy (haricot) (Phaseolus vulgaris L.) bean meal, or the heated meal supplemented with one of two fractions from raw navy beans: F3 was a preparation of navy bean trypsin inhibitor; F4 was a toxic factor known to depress the growth of rats. After 2 weeks, body-weights and pancreas weights were determined and proteolytic and amylolytic enzymes in the pancreas were assayed. The contents of the small intestine were collected and analysed for nitrogenous compounds.
2. In germ-free chicks final body-weights were not affected by F3 and only slightly depressed by F4. A significant depression of 12% occurred when raw meal was given. In conventional chicks depressions in final body-weights of 13·5, 26·5 and 48% were recorded with F3, F4, and raw meal respectively.
3. In birds given either raw meal of F3, there was an increase in size of the pancreas and in its concentration of trypsin, but a reduction in concentrations of chymotrypsin and amylase. The concentration of nitrogen in the insoluble fraction of the intestinal contents was increased by raw meal and F3 in both environments.
4. It was concluded that the presence of the gut microflora aggravated the growth-depressing effect on chicks of raw navy-bean meal. In the absence of the gut flora neither factor had any effect on growth. Possible explanations are discussed.