In order to explore the role of buried lysine residues of bovine serum albumin (BSA) in its interaction with bromophenol blue (BPB), three acetylated derivatives of albumin namely: 90%, 100% and 10%/chiefly having modification of buried lysine residues) were prepared by conventional and double modification techniques. The modification of lysine residues resulted in the change in conformation, as evidenced by the increase in Stokes radius from 3.55 nm (for native albumin) to 4.91 and 4.97 nm for 90% and 100% acetylated albumins, respectively. Modification of buried lysine residues (10% acetylated preparation) of albumin increased the Stokes radius up to 3.96 nm. The interaction of BPB with albumin preparations was studied spectrophotometrically at ionic strength 0.4 and at three different pH values i.e., 4.0, 6.0 and 8.0. There was decrease in BPB binding on increasing the modification. A decrease of 63% and 69% was noticed at pH 8.0 in 90% and 100% acetylated preparation, respectively. The 10% acetylated BSA preparation with minimum conformational changes also showed a significant decrease (31%) in BPB binding at pH 8.0. The change in Kd from 2.04 x 10(-6) M for native albumin to 5.41 x 10(-6) M for 100% acetylated albumin and 3.39 x 10(-6) M for 10% acetylated preparation at pH 8.0 confirmed the critical role of buried lysine residues in BPB-BSA interaction.