The interaction of chromazurol S (CAS) with bovine serum albumin (BAS) has been investigated by a spectrophotometric method, based on the fact that the absorption spectra of the free CAS and the combined CAS on BSA are different. It is considered that the combination of BSA and CAS is mainly due to electrostatic force. The Scatchard model, which has been widely used previously to describe the interaction between small mols. and proteins, is not suitable in the treatment of data obtained here. A new linear regression equation has been proposed, by which, the combination constant, the max. combination no. of CAS on one BSA mol., and the molar absorptivity of the combined CAS can be calculated The effect of experimental conditions on the solution equilibrium has also been examined, and it has been found that acidity and concentration of sodium chloride can give a considerable influence on the combination equilibrium

Key words: SPECTROPHOTOMETRY, ABSORPTION SPECTROMETRY, CHROME AZUROLS, SERUM ALBUMIN