The cysteine-rich LIM motif is highly conserved between invertebrates and mammals. This motif shows similarity both to proteins that bind zinc and to ferredoxins, which contain iron-sulfur clusters. Two tandem copies of the LIM motif are found in a number of presumptive transcription factors, including the protein product of the Caenorhabditis elegans cell-lineage gene lin-11. To investigate the possible metal-binding properties of the LIM region of the lin-11 protein, we expressed and purified a 151-amino acid peptide containing the tandem LIM motifs. The purified peptide binds both zinc (two atoms per protein molecule) and iron (as a redox-active iron-sulfur cluster, with four atoms of iron and four atoms of inorganic sulfide per protein molecule). These observations suggest that the LIM motif is a metallodomain that might function in a redox-sensitive regulation of transcription.