We tested whether the expression of melanin-based plumage traits reflects their bearer's levels of circulating antioxidants by repeatedly injected molting juvenile male House Sparrows (Passer domesticus) with a low or high dose of a substance (DL-buthionine-S,R-sulfoximine, hereafter BSO) that reduces the levels of cellular glutathione, a key antioxidant that inhibits melanin production. We predicted that birds with progressively lower glutathione levels would produce a progressively larger bib, a melanin-based trait present in males and related to their competitive ability. These injections were combined with dietary calcium supplements because we had found in a previous study that dietary calcium had a negative effect upon bib size in this species, perhaps because calcium level and glutathione level covary. We found that bib size was negatively related to dietary calcium but was unaffected by BSO injections. We also found no effect of BSO injections on the reflectance of bib feathers, although there was an interactive effect with dietary calcium, and no effect upon the size or reflectance of structurally based plumage traits. Bib size was not related to body condition at the time the bib was first being produced, nor was it related to the levels of circulating glutathione or total antioxidants. Evidence suggested that BSO injections reduced glutathione but that the effects were short-lived, possibly explaining why we found no effect on bib size. In sum, we found no evidence that bib size in House Sparrows is related to glutathione levels but confirmed that it is negatively related to dietary calcium.