The invention relates to chimeric proteins comprising an artificial sequence for cleavage by thrombin, in which the activation peptide is fibrinopeptide A. Preferably, said chimeric proteins are derived from the zymogen of a serine protease such as PC or FX, by replacing the activation peptide of said zymogen with fibrinopeptide A, or a portion thereof, comprising at least amino acids P.sub.10 to P.sub.1 of said fibrinopeptide.