We have examined in vitro radical trapping by the monoamine oxidase-B (MAO-B) inhibitor deprenyl and compared it to the specific MAO-B inhibitor MDL 72,974A. The capacity for the compounds to prevent .OH-mediated oxidation of biological substrates was examined by determining their ability to inhibit oxidation of 2-deoxyribose and phosphatidylcholine liposomes using the thiobarbituric acid reactive substances (TBARS) assay. MDL 72,974A gave a dose-dependent inhibition of 2-deoxyribose oxidation, while deprenyl generated a strong false positive TBARS reaction with both the sugar and the liposomes. When lipid peroxidation was monitored by conjugated diene formation, deprenyl inhibited oxidation while MDL 72,974A was without effect suggesting that trapping of .OH was not responsible for activity. Deprenyl inhibited liposomal peroxidation initiated with the water-soluble peroxyl radical generator 2,2'-azobis (2-amidinopropane) (ABAP) with an IC50 of 78 microM as compared to 4.2 mM for MDL 72,974A. A similar difference was observed using the lipophilic peroxyl radical generator 2,2'-azobis (2,4-dimethylvaleronitrile) (AMVN). The data indicate that radical trapping by the MAO-B inhibitors provides differential protection against biological substrates and may involve trapping of secondary peroxyl radicals rather than .OH.