Malonaldehyde is one of the end products of lipid oxidation, and because of its bifunctionality it can produce a conjugated Schiff base with a primary amine. The fluorescence properties, the reactivity to thiobarbituric acid (TBA) and the antioxidative activity of conjugated Schiff bases of malonaldehyde and aniline (I₁), p-toluidine (I₂), p-anisidine (I₃), p-chloroaniline (I₄) and p-aminoacetophenone (I₅) were investigated. Compounds I₁-I₅ showed no significant intrinsic fluorescence. The unstable compound I₅ produced highly fluorescent compound(s) in a mixture of dimethylformamide-tetra-n-propylammonium hydroxide. In the TBA reaction, compounds I₁-I₄ produced less than 30% as much a red color as malonaldehyde, while the unstable compound I₅ produced as intense a red color as malonaldehyde. Compounds I₁-I₄ were active as antioxidants with methyl oleate, lard and soybean oil.