Faffa, the commercially produced infant formula in Ethiopia, contains approximately 20% protein derived from wheat, soy flour, chick peas and skimmilk. The paper examines the effect of substituting the chick peas in Faffa with haricot beans. Protein efficiency ratio (PER) was drastically reduced due to the substitution. Pre-toasting the haricot beans improved the PER value of the Faffa to the level of that of chick peas Faffa. The essential amino acid patterns of the two preparations of Faffa were found to be similar and therefore could not explain the difference in PER. It was suspected that improvement of PER by heat treatment of the haricot bean Faffa was due to inactivation of the trypsin inhibitor. Analysis for trypsin inhibitor content confirmed that the lower PER value for Faffa prepared from untoasted Faffa is due to higher trypsin inhibitor content of haricot beans as compared to chick peas.