The interactions of bromcresol green with various components isolated from heat treated bovine serum albumin at 65 degrees C and pH 9.0 were studied spectrophotometrically. The difference spectra of the dye induced by native albumin and component 1, which has the same electrophoretic mobility as native albumin, had a trough at 616 nm and peak at about 668 nm, whereas those induced by the denatured monomer (component 1'), denatured dimer (component 2) and denatured trimer (component 3) had a trough at 610 nm and peak at 655 nm. From these difference spectra, it was concluded that component 1 is a mixture of at least two components (native and modified native forms) and that components 1', 2 and 3, formed during the process of denaturation, have essentially the same conformation in the environment of the binding sites for bromcresol green. A possible mechanism for the heat denaturation of serum albumin is proposed.