During antioxidant screening using 1,1-diphenyl-picrylhydrazyl (DPPH) and a lipid peroxidation assay, a streptomycete strain was found to produce herbimycin A and dihydroherbimycin A as antioxidants in the culture filtrate. These molecules were identified by using spectral analyses, including infrared, ultraviolet, mass spectrum, and nuclear magnetic resonance assays. In the DPPH radical-scavenging assay, dihydroherbimycin A exhibited more potent antioxidant activity (IC50, 1.3 mu M) than alpha-tocopherol (IC50, 2.7 mu M) that was used as a reference compound. In the lipid peroxidation assay, both herbimycin A and dihydroherbimycin A demonstrated antioxidant activities of 61% and 72%, respectively, at 100 mu g/ml, while alpha-tocopherol exhibited an activity of 93% at the same concentration. Therefore, dihydroherbimycin A might have the potential to be developed into a new therapeutic agent.