Enzymatic hydrolysates of Laminaria japonica were evaluated for antioxidative activities using hydroxyl radical scavenging activity and protective effects against $H_2O_2$-induced DNA and cell damage. In addition, activities of antioxidative enzymes, including catalase, glutathione peroxidase, and glutathione S-transferase, of the enzymatic hydrolysates from L. japonica were also estimated. L. japonica was first enzymatically hydrolyzed by seven carbohydrases ($Dextrozyme^{(R)}$, $AMG^{(R)}$, $Promozyme^{(R)}$, $Maltogenase^{(R)}$, $Termamyl^{(R)}$, $Viscozyme^{(R)}$, and $Celluclast^{(R)}$ [all from Novo Co., Novozyme Nordisk, Bagsvaerd, Denmark]) and five proteinases ($Flavourzyme^{(R)}$, $Neutrase^{(R)}$, $Protamex^{(R)}$, $Alcalase^{(R)}$ [all from Novo Co.], and pancreatic trypsin). The hydroxyl radical scavenging activities of Promozyme and pancreatic trypsin hydrolysates from L. japonica were the highest as compared to those of the other carbohydrases and proteinases, and their 50% inhibitory concentration values were 1.67 and $317.49;{mu}g/mL$, respectively. The pancreatic trypsin hydrolysates of L. japonica exerted a protective effect on $H_2O_2$-induced DNA damage. We also evaluated the protective effect on hydroxyl radical-induced oxidative damage in PC12 cells via propidium iodide staining using a flow cytometer. The AMG and pancreatic trypsin hydrolysates of L. japonica dose-dependently protected PC12 cells against cell death caused by hydroxyl radical-induced oxidative damage. Additionally, we analyzed the activity of antioxidative enzymes such as catalase, glutathione peroxidase, and the phase II biotransformation enzyme glutathione S-transferase in L. japonica-treated cells. The activity of all antioxidative enzymes was higher in L. japonica-treated cells compared with the nontreated cells. These results indicate that enzymatic hydrolysates of L. japonica possess antioxidative activity.