Effects of the modification of casein components by malonaldehyde were investigated. Ultraviolet and fluorescence spectra, the determination of amino groups, SDS-polyacrylamide gel electrophoresis and amino acid analysis indicated that malonaldehyde reacted with lysine residues of protein. The reaction proceeded in two steps: the formation of enamine structure (O=CHCH=CHNHR) and then the formation of amino-iminopropene structure (RN=CHCH=CHNHR) with the characteristic fluorescence spectra. Ultracentrifugal experiments in the absence of calcium ion revealed that self-association and complex formation of casein components were impaired by the modification. Properties of casein components in the presence of calcium ion, precipitation and micelle formation, were lost by the modification. αs1-Casein modified by malonaldehyde was not readily hydrolyzed by proteinases. On the basis of the results, it is concluded that amino-iminopropene formation leads to the deterioration of proteins. Biological significance of the modification of protein by malonaldehyde is also discussed.