Blood coagulation can be initiated when factor VII(a) binds to its cofactor tissue factor. This factor VIIa/tissue factor complex proteolytically activates factors IX and X, which eventually leads to the formation of a fibrin clot. Plasma contains a lipoprotein-associated coagulation inhibitor (LACI) which inhibits factor Xa directly and, in a Xa-dependent manner, also inhibits the factor VIIa/tissue factor complex. Here we report the cloning of the human LACI gene and the elucidation of its intron-exon organization. The LACI gene, which spans about 70 kb, consists of nine exons separated by eight introns. As has been found for other Kunitz-type protease inhibitors, the domain structure of human LACI is reflected in the intron-exon organization of the gene. The 5' terminus of the LACI mRNA has been determined by primer extension and S1 nuclease mapping. The putative promoter was examined and found to contain two consensus sequences for AP-1 binding and one for NF-1 binding, but no TATA consensus promoter element.