DNA vaccination has emerged as a powerful approach in the search for a more efficacious vaccine against tuberculosis (TB). In this study, we evaluated the immunogenicity and protective efficacy of Mtb8.4/hIL-12 chimeric gene vaccine. The Mtb8.4/hIL-12 chimeric gene was amplified by PCR and cloned into the eukaryotic expression vector pCI-neo. C57BL/6N mice were vaccinated with Mtb8.4/hIL-12 chimeric gene vaccine for three times at 3 weeks intervals. Four weeks after the final inoculation, three mice per group were sacrificed to assess cytokine response and CTL induction and the other five mice per group were challenged intravenously in a lateral tail vein with 1 x 10(6) CFU of virulent Mycobacterium tuberculosis H37Rv. Spleen and the left lung were harvested from each mouse at 4 weeks after infection and homogenized in sterile saline. Serial dilutions of organ homogenates were plated on L-J agar and incubated 37 degrees C until colonies were visible 4 weeks later. Protective efficacies in each experiment were expressed as reduced CFU and were compared with the negative control group. The right lung was obtained from each mouse and immediately inflated with and stored in 10% formalin saline. Tissues were embedded in paraffin, sectioned and stained with hematoxylin and eosin. Mtb8.4/hIL-12 chimeric gene vaccine induced the secretion of more of Th1 cytokines, but not IL-4 and enhanced CTL activity. Mice immunized with Mtb8.4/hIL-12 chimeric gene vaccine had fewer and smaller tubercles than control groups. As expected, control mice had the highest bacterial loads in both lung and spleen. Immunization with Mtb8.4/hIL-12 chimeric gene vaccine could remarkably reduced CFU counts in organs. When it was used to construct the chimeric gene vaccine, hIL-12 could improve the immune efficacy of Mtb8.4 gene vaccine.