The dependence of the slopes of normalized Perrin plots on the excitation wavelength was established. It was shown that the cause of this effect is the anisotropy of the Brownian rotation of proteins, which must be regarded as asymmetrical particles with specific, but not random, orientation of tryptophan with respect to the main axes of the macromolecule. These findings were analysed on the basis of the rotational depolarization theory of such systems, applied for the case when bands of two absorption oscillators overlap as it is for oscillators 1La and 1Lb in the longest wavelength absorption band of tryptophan. It was shown that anisotropy of Brownian molecular rotation is one of the factors that determines the form of the polarization spectrum. The difference of the polarization spectrum of proteins from that of tryptophan, extrapolated to the infinite viscosity, is determined by energy transfer processes in proteins.