β-Carboline alkaloids including harmaline have been shown to inhibit enzymatically or nonenzymatically induced-lipid peroxidation of microsomes. This study was done to explore the antioxidant ability of harmaline and harmalol on the oxidative injuries of hyaluronic acid, lipid and collagen by Fe²⁺ and H₂O₂. Their scavenging actions on reactive oxygen species were also examined. Harmaline, harmalol, superoxide dismutase, catalase and DMSO inhibited both degradation of hyaluronic acid by Fe²⁺ and H₂O₂ and lipid peroxidation of microsomes by Fe²⁺. In these reactions, DABCO inhibited degradation of hyaluronic acid but did not affect lipid peroxidation. β-Carbolines inhibited degradation of cartilage collagen by Fe²⁺, H₂O₂ and ascorbic acid. The reduction of ferricytochrome c due to autoxidation of Fe²⁺, which is inhibited by superoxide dismutase, was not affected by harmaline and harmalol. They also did not have a decomposing action on H₂O₂. Hydroxyl radical production in the presence of Fe²⁺ and H₂O₂ was inhibited by harmaline, harmalol and DMSO. Harmaline and harmalol may inhibit the oxidative injuries of hyaluronic acid, lipid and cartilage collagen by Fe²⁺ and H₂O₂ through their scavenging actions on reactive oxygen species, OH and probably iron-oxygen complexes and exert antioxidant abilities.