Our previous findings suggested that coagulation factor XFriuli could be functionally defective owing to a point mutation in the portion of the factor X gene coding for the fully activated heavy chain. To verify the existence of this postulated change, we analyzed all eight exons of the normal and Friuli factor X gene. Each exon was amplified from genomic DNA using the polymerase chain reaction and cloned into the plasmid pUC19. The amplified DNA inserts were subjected to direct sequencing by the dideoxy chain termination method with forward and reverse oligonucleotide sequencing primers. A point mutation (C to T transition at nucleotide position 19,297) that results in coding for serine (TCC) in place of proline (CCC) at amino acid position 343 was found. This substitution involves a highly conserved proline residue oriented spatially close to both the cleavage site of the zymogen and the active site of the enzyme and explains the previous observations of discrete biochemical and functional differences between factor XFriuli and normal factor X. The mutation abolished an HgiCI restriction site present in the normal factor X gene, and this change constitutes the basis for a convenient method for screening individuals carrying this molecular defect. Proline343 is in conserved region 5 of the serine protease superfamily to which factor X belongs and is part of a 14- residue L*****P******C motif that occurs in at least 16 other enzymes. Computer analysis suggests that the motif may be an essential aspect of conformational features important to functional properties of factor X as well as other serine proteases.