Two outer-membrane proteins are involved in the uptake of iron from transferrin by certain Gram-negative bacteria, transferrin-binding proteins 1 and 2. The gene encoding transferrin-binding protein 1 from a serotype 1 isolate of the Gram-negative pathogen Actinobacillus pleuropneumoniae was cloned, and a fragment encoding 700 amino acids of Tbp1 was expressed in Escherichia coli. We also report here sequencing of the tbp1 gene and a comparison of the deduced amino acid sequence with Tbp1s from related species. The predicted polypeptide product of tbp1 is a 106 kDa protein with a 22-residue signal peptide.