Cold　atmospheric　pressure　plasma　is　widely　used　in　research　for　biomedical　applications　and　is　a　promising　therapy　to　selectively　eradicate　cancer　cells.　However,　fundamental　information　related　to　the　plasma　modification　of　biomolecules　in　aqueous　solution　remains　elusive.　In　this　work,　we　studied　the　chemical　basis　of　tryptophan　(Trp)　oxidation　using　an　Ar　plasma　jet　and　investigated　the　effects　of　plasma-treated　tryptophan　(PTT)　on　cell　viability.　The　results　show　that　the　main　product　of　the　Trp　of　plasma　induced　oxidation　is　a　mixture　of　hydroxyl　derivatives　and　hydroperoxides　in　aqueous　form.　The　products　result　primarily　from　the　hydroxyl　radical　(center　dot　OH)　attacking　the　Trp,　which　can　be　explained　by　the　interaction　with　both　the　aromatic　and　the　pyrrole　rings.　We　observed　that　the　PTT　has　a　different　proliferation　effect　between　the　growth　of　the　B16　melanoma　cells　and　the　L929　fibroblast　cells.　The　experimental　results　indicated　that　the　effect　of　the　PTT　is　dose-dependent　in　the　B16　cells,　ranging　from　cell　proliferation　to　cytotoxic　damage　with　apoptosis.　Furthermore,　we　examined　the　intracellular　changes　in　hydrogen　peroxide　(H2O2)　and　superoxide　radicals　(O-2(center　dot-))　following　the　Trp-derived　treatment　using　the　fluorescence　probe　method.　Rigorous　identification　of　the　reactive　oxygen　species　(ROS)　produced　by　PTT　in　cells　as　O-2(center　dot)　and　H2O2　helped　establish　the　cellular　source　of　ROS.　It　is　shown　that　the　intracellular　H2O2　might　originate　from　the　activation　of　NADPH.　These　results　suggested　that　the　amino　acid　and　protein　hydroperoxides　may　play　a　crucial　role　in　the　action　of　plasma　on　the　biologic　target.