We investigated the transfer of three antioxidants - melatonin, S-adenosyl methionine (SAM) and various forms of vitamin E - across the term, normal human placenta. The transport technique involved the single, isolated placental cotyledon system in vitro. Melatonin crossed the placental rapidly, equally to the freely diffusible marker, antipyrine. There was no biotransformation of the agent. SAM was transferred slowly, similarly to passively transported L-glucose as a marker. There was a breakdown of SAM to at least one other derivative; the process appeared to be nonenzymatic. Vitamin E was transferred slowly, at a rate only 10% of L-glucose. The natural RRR (nonracemic) form of vitamin E was transported best. Free vitamin, rather than the acetate seems to be transferred best, a finding that will require further study.