A recombinant human arginase I (liver arginase) covalently attached, via a succinamide propionic acid (SPA) linker, to a polyethylene glycol (PEG) of molecular weight 5,000 [rhArg-peg(5,000mw)] with potential antineoplastic activity. Upon intravenous administration of pegylated recombinant human arginase I BCT-100, arginase metabolizes the amino acid arginine to ornithine and urea, depleting intracellular arginine, which may inhibit proliferation of cells that are auxotrophic for arginine such as hepatocellular carcinoma (HCC) cells. This agent may also work synergistically with various cytotoxic agents.