Recombinant porcine mature interleukin-18 (rPomIL-18) has been purified from the haemolymph of silkworms. After co-infection of two recombinant baculoviruses (BmAcpVL 1392-IL-18-His and BmAcpVL1 392-case-1) into the silkworm, rPomIL-18 was produced and secreted into the haemolymph. Polyethylene glycol (PEG) 6000 was added to the haemolymph at 8% (v/w) to precipitate storage proteins which would otherwise bind non-specifically to the metal chelating column and the supernatant then was applied to Sepharose bonded with Ni2+. rPomIL-18 was eluted from the column using 100 mm imidazole buffer at pH 8 with a purity of 93.6%. Approximately 5.3 mg purified rPomIL-18 was obtained from 22 ml haemolymph. It could induce interferon-gamma formation from porcine peripheral blood mononuclear cells.