To examine the antioxidative compounds of nonprotein amino acids in a red alga, Porphyra yezoensis, an ethanol extract of the cultured thalli was fractionated with Dowex columns. The basic fraction V showed strong antioxidative capacities with ferric thiocyanate and TBARS measurements. In this basic fraction, histidine, 3-methylhistidine, carnosine, and anserine were detected beside ornithine, lysine, and arginine by amino acid analysis. The occurrence of carnosine and anserine suggests that the histidine-related compounds also contribute to the antioxidative reactions in P. yezoensis.