The effect of rapid freeze-thawing on the structural rearrangement of integral and peripheral membrane proteins was studied using fluorescent spectroscopy. It was discovered that a long wave shift occurred in the fluorescence spectra of cytochrome P450, both in solution and within phospholipid vesicles. The efficiency of quenching of membrane-bound cytochrome P450 with acrylamide also increased after freezing. Similarly, a long wave shift in the fluorescence, spectra occurred after freezing spectrin solution and erythrocyte ghosts. These data are interpreted in terms of physical and chemical factors in the medium affecting the spectral properties of protein chromophores, and suggest that an increase occurs in the polarity of the microenvironment of the protein molecules, resulting in conformational modification. Study of the temperature dependence of spectrin fluorescence in solution, and within erythrocyte ghosts, suggests that modification of protein dynamics occurs both during freezing and during thawing. Despite various differences in the structure of the protein molecules, and in the phospholipid environment and solvent composition, freeze-thawing results in unilateral, irreversible conformation changes, more prominent in peripheral than integral proteins. The possibility that these changes contribute to cryoinjury to cell membranes is discussed.