Theoretical analysis was performed for the C-13 chemical shifts of the retinal chromophore in bacteriorhodopsin (bR) by means of ab initio NMR shielding calculation, based on the localized orbital/local origin method. In order to comprehensively investigate the correlation between the C-13 chemical shieldings of the unsaturated carbons and physicochemical perturbations relating to the spectral tuning of bacteriorhodopsin, the following three factors are taken into account in the present calculation : (1) change in strength of the hydrogen bonding between protonated retinylidene Schiff base and its counterion, (2) conformational changes about single bonds of the conjugated chain, and (3) electrostatic interactions between the Schiff base and electric dipoles. On the basis of these calculations, we successfully find a molecular model for which the shielding calculation almost completely reproduces the observed chemical shift data for the chromophore of bR.