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The binding of alizarin yellow G--an azo derivative of salicylic acid--by bovine serum albumin has been investigated using the method of equilibrium dialysis. Six strong and a number of additional, weak binding sites have been found to be present. The system is characterized by strong positive cooperativity between the first and second sites. Six binding constants have been determined on the basis of a simplified mathematical model. The results are approximately 2 X 10(4) M-1 for the first binding site, 6 X 10(5) M-1 for the second, and between 4 X 10(4) and 10(5) M-1 for the rest. The phenomenon is discussed in terms of the existence of various conformers or of the conformational adaptability of albumin. Cobinding by salicylic acid does not displace alizarin yellow G but induces a conformational change in the protein which affects the absorption spectrum of the bound dye. As expected for this kind of heterotropic interaction, the spectrum of the system albumin-salicylic acid is simila rly affected by the cobinding of alizarin yellow G.