Addition of increasing concentrations of hydrogen peroxide (H2O2) caused progressive decreases in dimethylthiourea (DMTU) concentrations which were inhibitable by simultaneous addition of catalase, but not the superoxide anion (O2-.) scavenger, superoxide dismutase (SOD), or hydroxyl radical (.OH) scavengers, such as mannitol, sodium benzoate or dimethyl sulfoxide (DMSO). In parallel, addition of increasing concentrations of H2O2 with FE++/EDTA (but not H2O2 alone) caused decreases in DMSO concentrations which were inhibitable by simultaneous addition of .OH scavengers but not SOD or catalase. Addition of DMTU, but not DMSO, also decreased H2O2 concentrations in vitro. The results indicate the relative scavenging specificities of DMTU and DMSO for H2O2 and .OH, respectively. The findings also suggest that measurement of DMTU or DMSO consumption could help assess the contribution of O2 metabolites in biological systems.