Irradiated tumor cells genetically modified to secrete granulocyte/macrophage-colony-stimulating factor (GM-CSF tumor vaccine) are potent stimulators of systemic antitumor immunity. For the preparation of a GM-CSF gene-modified tumor vaccine, it is important to achieve efficient genetic transduction of tumor cells, leading to an appropriate expression of the induced gene. In this report, with a view to developing a protocol for an effective cancer vaccination therapy, we examined the vaccination efficacies of tumor cells secreting GM-CSF by either adenovirus- or retrovirus-mediated genetic transduction. By using an adenoviral vector, Adex1CAmGMCSF, a highly efficient gene delivery and a high-level expression of the GM-CSF gene were achieved. Unexpectedly, animal vaccination studies showed that the GM-CSF tumor vaccine transduced with the Adex1CAmGMCSF recombinant adenovirus (adenoviral GM-CSF tumor vaccine) was less efficacious than that transduced with the MFGmGMCSF recombinant retrovirus (retroviral GM-CSF tumor vaccine). The GM-CSF serum concentration attained by the adenoviral GM-CSF tumor vaccine was much higher than that obtained by the retroviral GM-CSF tumor vaccine. Our findings indicate that an optimal level of GM-CSF production is important for the tumor vaccine to elicit an adequate response in the host antitumor immunity.