We have recently discovered a reaction that all antibodies, regardless of source or antigenic specificity can catalyze, that is the reaction between singlet dioxygen ((1)O(2)(*)) and H(2)O to generate H(2)O(2). We have named this process the antibody-catalyzed water oxidation pathway (ACWOP). As part of our ongoing investigations into the possible biological role of this pathway, we have studied whether isoalloxazine-containing cofactors, that are known to be endogenous photosensitizers via Type-II pathways to generate (1)O(2)(*), such as riboflavin (RF, Vitamin B2) can trigger the ACWOP. Herein we show that regardless of the antigenic specificity or heavy and light chain composition, all antibodies and their fragments are able to intercept the (1)O(2)(*) generated by photo-oxidation of RF in the presence of oxygen (ambient aerobic conditions) to activate the ACWOP. The initial rate of HOOH generation by a panel of murine antibodies ranges from 0.218 to 0.998 microM/min. The initial rate of antibody-catalyzed HOOH production is accelerated in D(2)O and is quenched in NaN(3), highlighting the key intermediacy of (1)O(2)(*) in the process. Critically, the ACWOP is photo-activated at physiologically relevant concentrations of RF (<50 nM) suggesting that this pathway may be relevant in an in vivo setting. Finally, when activated by RF the ACWOP generates oxidants that accelerate the hemolysis of sheep RBCs hinting at a pathophysiological effect of this RF-induced photo-oxidation pathway.