OBJECTIVE To examine the expression of recombinant cytolethal distending toxin (CDT) produced by Actinobacillus actinomycetemcomitans (Aa). METHODS CDT encoding gene cdtABC was amplified by PCR. Through TA clone and restriction endonuclease digestion, gene cdtABC and vector pQE60 were ligated to form pQE60-cdtABC expression system which transformed into competent cells. Protein expression was induced by IPTG and examined by SDS-PAGE and Western-blotting. RESULTS Random colony PCR of pQE60-cdtABC transformed cells demonstrated that all strains contained cdtABC gene. The DNA sequence was blast with cdtABC gene from GenBank and 99% homology was obtained. SDS-PAGE and Western-blotting confirmed that recombinant CDT was obtained. CONCLUSIONS CDT protein expression system was reconstructed and recombinant protein was obtained. Actinobacillus actinomycetemcomitans;