The interaction of ferritin mRNA is regulated by iron via the interaction of a cytoplasmic binding protein (IRE-BP) with a specific stem-loop structure in the 5' untranslated region (UTR), referred to as the iron-responsive element (IRE). A high affinity RNA-protein complex between the IRE and the IRE-BP functions as a repressor of translation in vivo. Translational repression appears to depend upon the position of the IRE in the 5' UTR of the mRNA. IREs located in the 5' untranslated region 67 nucleotides or more downstream of the 5' terminus of the mRNA fail to mediate iron-dependent translational regulation and give rise to constitutively derepressed transcripts. A model is proposed in which translational regulation of protein biosynthesis involves a position-dependent interference of the IRE/IRE-BP complex with one of the initial steps in translation initiation.