Although dendritic cells (DC) originate from bone marrow, they were not observed in fresh preparations of bone marrow cells (BMC). Likewise, accessory activity was barely measurable in a sensitive assay for this potent function of DC. However, both DC and accessory activity developed when BMC were cultured for 5 d. Based on fractionation before culture, nearly all of the accessory activity could be attributed to only 5% of the total BMC recovered in a low-density (LD) fraction. The LD-DC precursors differed from mature DC in a number of important respects. Removal of Ia+ cells from the LD fraction by panning did not decrease the production of DC when the nonadherent cells were cultured. Thus, the cell from which the DC is derived does not express or minimally expresses Ia antigens, in contrast to the strongly Ia+ DC that is produced in bone marrow cultures. Irradiation of LD cells before culture prevented the development of DC. When irradiation was delayed by daily intervals, progressive increases in the number of DC resulted, up to the fifth day. These findings, together with preliminary autoradiographic data, indicate that cell division has occurred, in contrast to the DC, which does not divide. We conclude that bone marrow-derived DC arise in culture from the division of LD, Ia- precursors.