In this work, the precipitation of porcine trypsin in solutions of volatile electrolytes formed by ammonia and carbon dioxide was investigated. The solubility curve of trypsin at 4°C was determined by measuring the compositions of the precipitate and supernatant phases. Analysis showed that ammonium carbamate is able to induce trypsin salting-out. The verified salting-out property of ammonium carbamate, the easy removal of the salt out of the protein precipitate and mother-solution and the high activity recovery demonstrate the feasibility of using ammonium carbamate as precipitant agent.