The affinity cleavage reagent Methidiumpropyl-EDTA-Iron(II) is applied to the structural analysis of 5S rRNA. Analysis of cleavage sites induced by MPE-Fe(II) on 5S rRNA shows that MPE intercalates easily between the unstable base pairs or into the bulges, thereby it strongly cuts the nucleosides nearby. The stable helical stems A, B, D and E as well as loop d are weakly cut. Most of the single-stranded loops are not cleaved. Based on the cleavage pattern of the 5S rRNA by MPE-Fe(II) and RNase V1, we suggest that MPE-Fe(II) may be used as a potential chemical probe in searching for the unstable helical regions of RNA, and for the sequences that appear to be involved in folding and distorting 5S rRNA.