A low level of chemilumnescence by hemoglobin (Hb) was detected in the reaction with H₂O₂ and hydrogen donors such as gallic acid and catechins. The photon intensity was affected by the ferric state of Hb (methemoglobin > oxyhemoglobin), and was roughly correlated with the radical-scavenging potential of catechins. We hypothesized the reversible activation reaction of Hb as the chemiluminescence mechanism of the H₂O₂/gallic acid/Hb system. It is indicated that the oxidized-Hb (Hb-OOH) formation was a chemiluminescence-rate–determining step and one-electron reduction by a hydrogen donor of the compound-I–type intermediate ([^·XFe^IV] = O) proved a chemiluminescence-specificity–determining step. Spectral analysis showed that the photon emission from the H₂O₂/gallic acid/Hb system was produced without singlet oxygen generation. The concentration dependence of photon intensity suggests a high consumption ratio of H₂O₂ leading to protection from H₂O₂ toxicity. Albumin was defined as a hydrogen donor by the isolation of chemiluminescent substance in plasma using this chemiluminescence system.