Extrahepatic bioartificial liver devices should provide an intact urea cycle to detoxify ammonia. The C3A cell line, a subclone of the hepatoma-derived HepG2 cell line, is currently used in this context as it produces urea, and this has been assumed to be reflective of ammonia detoxification via a functional urea cycle. However, based on our previous findings of perturbed urea-cycle function in the non-urea producing HepG2 cell line, we hypothesized that the urea produced by C3A cells was via a urea cycle-independent mechanism, namely, due to arginase 11 activity, and therefore would not detoxify ammonia. Urea was quantified using N-15-ammonium chloride metabolic labelling with gas chromatography-mass spectrometry. Gene expression was determined by real-time reverse transcriptase-PCR, protein expression by western blotting, and functional activities with radiolabelling enzyme assays. Arginase inhibition studies used N-omega-hydroxy-nor-L-arginine. Urea was detected in C3A conditioned medium; however, 15N-ammonium chloride-labelling indicated that N-15-ammonia was not incorporated into N-15-labetled urea. Further, gene expression of two urea cycle genes, ornithine transcarbamylase and arginase 1, were completely absent. In contrast, arginase II mRNA and protein was expressed at high levels in C3A cells and was inhibited by N-omega-hydroxy-nor-L-arginine, which prevented urea. production, thereby indicating a urea cycleindependent pathway. The urea cycle is non-functional in C3A cells, and their urea production is solely due. to the presence of arginase 11, which therefore cannot provide ammonia detoxification in a bioartificial liver system. This emphasizes the continued requirement for developing a component capable of a full repertoire of liver function.