The trace number of dendritic cells (DCs) present in tissues has limited the study of DC biology and development of clinical applications utilizing DCs. Here we show that hydrodynamics-based gene delivery of naked DNA encoding secreted human flt3 ligand (hFLex) can dramatically increase the number of functional DCs and natural killer (NK) cells. After a single injection of the hFLex gene, hFLex levels in mouse serum reached approximately 40 microg/ml and remained above 1 microg/ml for 5-6 days. Sustained levels of serum hFLex correlated with significant increases in the size of the lymphoid organs and in the proportion of dendritic cells and NK cells in both lymph nodes and spleen. The increase in DC and NK cell numbers started from day 5, and reached peak levels between day 8 and day 12. The levels then returned to normal on day 20. These DCs and NK cells were functional as evidenced by mixed leukocyte reactions and lysis of YAC-1 cells, respectively. These results suggest that delivery of the hFLex gene provides a simple, efficient, and inexpensive way of increasing DC and NK cell populations in vivo, and may have broad applications in the further study of DC and NK cell biology and in the development of immunotherapy strategies.