Crystalline amino acids (AA) are widely used in diets for pigs to either lower the concentration of soybean meal (SBM) in the diets or to balance the AA profile of alternative protein sources including high protein corn protein.  However, results of recent experiments demonstrated that even when diets are balanced for all indispensable AA, the excess levels of Leu in corn protein results in reduced growth performance of pigs.  This is likely due to interactions between Leu and Val, Ile, and Trp and it is, therefore, possible to partly restore the lost performance obtained for diets with corn protein if extra Val, Ile, and Trp is included.  However, it appears that pigs fed diets based on corn and SBM have better growth performance than those fed corn protein and crystalline AA.  It has also not been possible to demonstrate that low crude protein diets that contain crystalline AA instead of some SBM have greater net energy than diets based only on corn and SBM.  It therefore appears that SBM may provide benefits to pigs in addition to the AA that are provided from SBM.