Dendritic cells (DCs) are pivotal in inducing immunity or alternatively downregulating immune reactivity. In humans, the opposing phenotypic subsets of CD11c(+)/CD123(-) "myeloid" DCs and CD123(+)/CD11c(-) "lymphoid" DCs have been proposed to orchestrate these immune responses. In this study we determined the absolute numbers of both subsets in three resting hematopoietic tissues by employing a novel flow cytometry method, eliminating processing steps and calculations based on mononuclear cell percentages. Internal bead standards along with the cells of interest were simultaneously acquired directly from unmanipulated whole blood specimens. We found significant differences (p < 0.001) between the mean absolute numbers of CD123(+)/CD11c(-) lymphoid DCs among the three sources, with the fewest present in peripheral blood (8.2/ micro l), about 50% more in cord blood (12.2/ micro l), and fivefold more in bone marrow (40.2/ micro l). Cord blood and bone marrow CD11c(+)/CD123(-) myeloid DC counts did not differ from each other (23.5/ micro l and 28.9/ micro l, respectively) but peripheral blood contained significantly fewer (15.5/ micro l, p = 0.006). CD11c(+)/CD123(-) DCs had significantly higher surface expression of HLA-DR (p < 0.001) in all three sources. To test for association with the DC subsets, T, B, and natural killer (NK) lymphocytes were also enumerated. In bone marrow only, significant correlations were found between the size of the CD123(+)/CD11c(-) lymphoid DC pool and NK cells (p = 0.0029) and B cells (p = 0.0033). These data support the hypothesis that a common CD123(+)/CD11c(-) DC, NK cell, and B cell progenitor is resident in marrow, and this cell may be identical to the common lymphoid progenitor previously described in mice and/or the human CD34(+)/Lin(-)/CD10(+) progenitor.