Based on the view that the efficacy of the immune system is associated with the maturation state of the immune cells, including dendritic cells (DC), we investigated the development and functional potential of conventional DC and plasmacytoid pre-DC (p-preDC) in spleen, thymus, and lymph nodes during mouse development. Both CD11c+ DC and CD45RA+ p-preDC were detected in small numbers in the thymus as early as embryonic day 17. The ratio of DC to thymocytes reached adult levels by 1 wk, although the normal CD8alpha+ phenotype was not acquired until later. Significant, but low, numbers of DC and p-preDC were present in the spleen of day 1 newborn mice. The full complement of DC and p-preDC was not acquired until 5 wk of age. The composition of DC populations in the spleen of young mice differed significantly from that found in adult mice, with a much higher percentage (50-60% compared with 20-25%) of the CD4-CD8alpha+ DC population and a much lower percentage (10-20% compared with 50-60%) of the CD4+CD8alpha- DC population. Although the p-preDC of young mice showed a capacity to produce IFN-alpha comparable with that of adult mice, the conventional DC of young mice were less efficient than those of their adult counterparts in IL-12p70 and IFN-gamma production and in Ag presentation. These results suggest that the neonatal DC system is not fully developed, and innate immunity is the dominant form of response. The complete DC system required for adaptive immunity in the mouse is not fully developed until 5 wk of age.