Three distinct subtypes of dendritic cells (DC) are present in mouse spleen, separable as CD4(-)8 alpha (-), CD4(+)8 alpha (-), and CD4(-)8 alpha (+) DC. We have tested whether these represent stages of development or activation within one DC lineage, or whether they represent separate DC lineages. All three DC subtypes appear relatively mature by many criteria, but all retain a capacity to phagocytose particulate material in vivo. Although further maturation or activation could be induced by bacterially derived stimuli, phagocytic capacity was retained, and no DC subtype was converted to the other, Continuous elimination of CD4(+)8(-) DC by Ab depletion had no effect on the levels of the other DC subtypes, Bromodeoxyuridine labeling experiments indicated that all three DC subtypes have a rapid turnover (half-life, 1.5-2.9 days) in the spleen, with none being the precursor of another, The three DC subtypes showed different kinetics of development from bone marrow precursors. The CD8 alpha (+) spleen DC, apparently the most mature, displayed an extremely rapid turnover based on bromodeoxyuridine uptake and the fastest generation from hone marrow precursors. In conclusion, the three splenic DC subtypes behave as rapidly turning over products of three independent developmental streams.