Plasma from certain preterm and term babies has recently been shown to contain low molecular mass iron (LMrFe) that can be chelated and measured in the bleomycin assay. The chemical nature of such iron, when detected in biological fluids, is still unclear with suggestions that it may not be a redox active form of iron until it becomes bound to bleomycin. Here we show that all plasma samples, from umbilical cord bloods, containing bleomycin-detectable iron, activated the enzyme aconitase in a cell-free system. It is, therefore, highly likely that LMrFe detected in the bleomycin assay is a biologically redox active form of iron.