The effect of sodium dodecyl sulfate (SDS) on the spectral properties of phenylalanine residues in bovine serum albumin was studied at neutral pH by second derivative spectrophotometry. It was found that phenylalanine residues in the interior of bovine serum albumin became almost completely exposed on the surface of the protein on formation of a so-called AD12 complex. This conformational change began to be significant when 4 mol SDS bound to bovine serum albumin. At higher concentrations of SDS, when so-called ADn and AD2n complexes were formed, phenylalanine residues were transferred to the hydrophobic region again. This might be due to the involvement of phenylalanine residues in micelle-like clusters. Change in the conformation of bovine serum albumin involving tryptophan residues was also measured. These studies demonstrate the value of second derivative spectrophotometry in studies on conformational change of proteins.