Abstract: Objective To construct a prokaryotic expression system containing the dense granule protein 4 （GRA4） of Toxoplasma gondii，purify the expressed protein and detect its immunogenicity. Methods The specific fragment of GRA4 gene was amplified by PCR. After subcloning the prokaryotic expression recombinant pET-GRA4，the expressed product was purified with His·BindTM affinity chromatography and analyzed by Western blot. BALB/c mice were immunized with the GRA4 recombinant protein，and the antibody IgG titer was detected by ELISA. Results The pET-GRA4 prokaryotic expression system was obtained. The MW of the expressed protein was Mr 40 000 and formed in inclusion body. After purification，the recombinant protein could be specifically recognized by the T.gondii infected rabbit serum. Mice immunized with the purified recombinant protein elicited high titer of IgG antibody. Conclusion The pET-GRA4 recombinant protein was successfully expressed and purified，which shows the immunogenicity.

Key words: Toxoplasma gondii, GRA4, Prokaryotic expression, Protein purification