A modified essential amino acid index equation incorporating a digestibility factor (MEAAI), and a chemical score index (CSI) were developed from the essential amino acid profile of 18 different food proteins hydrolysed by the combined but sequential action of papain and pronase E enzyme systems. The essential amino acid composition of whole egg was used as a reference standard. The following equations were proposed: log MEAAI = 1/8 log (100 a/ae X DFP/DFe) + log (100 b/be X DFP/DFe) + .... + log (100 h/he X DFP/DFe) where a-h are the respective values of each of the 8 essential amino acids in the sample; DFp is the digestibility factor for food protein sample; DFe is the digestibility factor for whole cooked egg; CSI = 100 a/ae, in which a is the value of the limiting amino acid in the sample, and ae is the corresponding value in whole egg. Both equations were used to calculate the biological value of different protein foodstuffs. There was a close agreement between values obtained using the equations and reported values for the same foodstuffs published in the literature. There was also a close correlation (r = 0.665) between data obtained using MEAAI and those obtained using CSI in estimating biological values, suggesting that either equation could be used reliably and solely to estimate protein biological value of foods.