Rat liver selenoprotein P contains 10 selenocysteine residues in its primary structure (deduced). It is the only selenoprotein characterized to date that has more than one selenocysteine residue. Selenoprotein P cDNA has been cloned from human liver and heart cDNA libraries and sequenced. The open reading frames are identical and contain a signal peptide, indicating that the protein is secreted by both organs and is therefore not exclusively produced in the liver. Ten selenocysteine residues (deduced) are present. Comparison of the open reading frame of the human cDNA with the rat cDNA reveals a 69% identity of the nucleotide sequence and 72% identity of the deduced amino acid sequence. Two regions in the 3' untranslated portion have high conservation between human and rat. Each of these regions contains a predicted stable stem-loop structure similar to the single stem-loop structures reported in 3' untranslated regions of type I iodothyronine 5'-deiodinase and glutathione peroxidase. The stem-loop structure of type I iodothyronine 5'-deiodinase has been shown to be necessary for incorporation of the selenocysteine residue at the UGA codon. Because only two stem-loop structures are present in the 3' untranslated region of selenoprotein P mRNA, it can be concluded that a separate stem-loop structure is not required for each selenocysteine residue.