Cultured CD34^pos bone marrow cells acquire the capacity to form dendritic processes in response to treatment with A23187 or rhCD40L. Cultures of CD34^pos bone marrow cells initially expanded for 6 days in the presence of rh-c-kit ligand, rhGM-CSF, and rhTNF- to induce numerical expansion. Cells were then centrifuged and replated in identical fresh medium except for the absence of rh-c-kit ligand, with individual wells also receiving CI A23187 or rhCD40L. Twenty hours after replating, cells were harvested and centrifuged, resuspended in fresh medium and transferrred onto polylysine-coated glass chamber slides, incubated for an additional 20 minutes at 37^oC, and photomicrographed (600×) using DIC (Nomarski) optics. Photomicrographs are representative of multiple CI and CD40L doses tested in two separate experiments: untreated cells (A); cells treated for 20 hours with 375 ng/mL CI A23187 (B); cells treated for 20 hours with 10 μg/mL rhCD40L (C). Note dendritic processes exhibited on cells in panels B and C that are largely absent in A. Similar results were observed after CI treatment at 180 ng or 750 ng/mL or rhCD40L at 2.5 or 40 μg/mL (not shown).