1. Raw soya-bean meal (RS) was fractionated into soya-bean lyophilized extract (SLE), soya-bean lyophilized residue (SLR), acid-precipitated proteins (APP) and whey proteins.
2. Trypsin (EC 22.214.171.124) and chymotrypsin (EC 126.96.36.199) inhibitors (TI) were soluble at pH 8 and remained soluble after the extract was acidified to pH 4·4. Except for whey, heating abolished, almost totally, their inhibiting activity.
3. Feeding SLE diet (high TI content) and APP diet (low TI content) resulted in growth depression below the RS level. Feeding the SLR diet resulted in an optimal growth. Feeding diets containing heated fractions improved the growth rate though not to the level observed with heated RS (HS) diet.
4. RS, SLE, APP and whey diets produced similar pancreatic enlargement which could be totally (RS, whey) or partially (SLE, APP) abolished by heating.
5. Feeding the RS diet reduced pancreatic amylase content. The factor responsible for this effect cofractionated with SLE and whey proteins.
6. Two groups of factors in the various diets were probably responsible for the elevation in pancreatic proteases. The first group were the heat-labile factors present in RS, SLE and whey whereas the second group resisted the heat treatment and were found in APP and SLR.
7. The results suggest that for optimal growth rate of rats, heat treatment should be given to the unfractionated soya-bean proteins rather than to the isolated fractions. The results further indicated that TI are not the only factors that can lead to pancreatic enlargement and changes in pancreatic enzymes composition.