The plasmon absorption peak of 31-nm gold particles in aqueous solution was redshifted and broadened by the adsorption of protein. This effect is more striking in the acidic solution than in the alkaline solution. The adsorption is irreversible. When a small amount of protein was added to the colloidal solution, this effect was weak, but a small shoulder emerged at about 680 nm. With increasing quantity of the protein, the effect of the interaction became very strong. The small shoulder became larger and shifted to about 720 nm. But further addition of protein reduced the effect and finally the absorption spectrum became identical with the original colloidal solution. A model is presented which considers two adsorption manners of the protein on gold. Cytochrome b-562 is of cylindrical shape (height is 5.0 nm and diameter is 2.5 nm.) and has the chromophore at the position of about 3.5 nm from the bottom. Consequently, the "side-on" adsorption manner of the protein on gold which dominates at low concentrations and the "tail-on" adsorption fashion which dominates at high concentrations cause difference in the interaction distance between the surface plasmon and chromophore. The interaction between gold particles and the protein depends largely on the distance between the gold-particle surface and protoheme IX in the protein. The redshift and broadening of absorption peaks occurred at the side-on adsorption manner. We disclose an initial effort for developing a novel method to determine the configuration of protein adsorbed on colloidal particles.