The expression of the different domains of a complex serine protease, recombinant Factor C of the horseshoe crab, Carcinoscorpius rotundicauda, was achieved using pClneo and pEGFP-N1 vectors. In an in vitro coupled transcription-translation assay, the truncated CrFC cDNA insert coding for the initial 331 amino acids expressed the expected 36 kDa polypeptide lacking the antigenic epitopes. The highly disulfide-bonded Cys-rich and EGF-like domains at the N-terminus of CrFC which bind LPS were fused to GFP. When expressed in COS-1 cells, this fusion protein did not alter the localisation of the chromophore and remained soluble, indicating that other domains could be responsible for the membrane-bound nature of the recombinant Factor C, rFC. The expression of this LPS-binding domain of rFC as a soluble protein suggests possibilities of obtaining active rFC in mammalian cells.