The cDNA encoding Carcinoscorpius rotundicauda Factor C, CrFC, when expressed in mammalian cells, produced full length and truncated variants of rFC. In an in vitro coupled transcription-translation system, aberrant products were generated from pCDNAI/CrFC21, as opposed to pClneo/CrFC21 which gave rise to the expected polypeptide. COS-1 cells transfected with pClneo/CrFC21 expressed Factor C-related proteins which differ from those produced in the yeast, Pichia pastoris. The expression of a complex, mosaic protein such as the rFC in mammalian cells also resembled other mammalian blood factors, indicating that such cells could be used for production of a second generation ’limulus lysate’.