S100A11 (calgizzarin or S100C), a member of the S100 family of Ca2+-binding proteins, was first identified in chicken gizzard smooth muscle and subsequently detected in several mammalian species and tissues. We now report the full-length coding sequence of avian smooth muscle S100A11. The cloned nucleotide sequence is 515 bases in length, which includes in-frame start and stop codons and encodes a protein of 101 amino acids. The chicken S100A11 sequence differs from human S100A11 at 25 positions (9 conserved) and is four residues shorter (overall identity 72.4%, similarity 81%). The protein contains two EF hands and conserved hydrophobic residues involved in dimer formation. Cloned avian S100A11 expressed in Escherichia coli and purified by Ca2+-dependent hydrophobic interaction chromatography and ion-exchange chromatography was recognized by polyclonal antibodies raised against tissue-purified protein and, like tissue-purified S100A11, bound 45Ca2+ in a gel overlay assay. Key words: S100A11, calgizzarin, Ca2+-binding protein, smooth muscle, avian.