Factor XIII, the clotting factor essential for covalent stabilization of the fibrin clot, is a heterodimer consisting of a2 and b2 subunits, with catalytic function residing in the a-chain. In order to address questions regarding sites of synthesis and chromosomal localization of the Factor XIII a-chain, cDNA was cloned from a lambda gt11 human placental cDNA library. Nucleotide and amino acid sequences were determined from the cDNA. Amino acid sequencing of purified platelet Factor XIII a-chains confirmed the authenticity of the lambda gt11 clone. The gene for Factor XIII a-chain was mapped uniquely to chromosome 6. Northern blot analysis of human placental and U937 (monocytelike) cell poly (A)+ mRNA showed a single approximately 4.0-kb message for the Factor XIII a-chain. These results provide conclusive evidence that the a-chain is synthesized by placenta and monocyte cell lines.