Patients with head and neck squamous cell carcinoma (HNSCC) have profound immune deficiencies. In 65% of these patients, there is an increased intra-tumoral presence of immune-suppressive CD34 ุ progenitor cells. The goal of the present study was to determine whether CD34 ุ cell levels were also increased in the peripheral blood of HNSCC patients and if these immune-suppressive cells could be differentiated into dendritic cells. Our results showed that CD34 ุ cell levels are increased in the peripheral blood of HNSCC patients. To assess if these CD34 ุ cells could differentiate into dendritic cells, they were isolated from the blood of HNSCC patients and cultured for 12 days with various cytokine combinations. Culturing CD34 ุ cells with stem cell factor (c-kit ligand) plus granulocyte-macrophage colonystimulating factor resulted in the appearance of a significant proportion of cells expressing phenotypic markers characteristic of dendritic cells. Also, including tumor necrosis factor-โฃ yielded a significant proportion of cells resembling the bipotential precursor cells for dendritic cells and monocytes (CD14 ุ CD1a ุ ), in addition to the dendritic-like cells. When the differentiation inducer 1โฃ,25-dihydroxyvitamin D 3 [1,25(OH) 2 D 3 ] was added along with the cytokine combinations, the yield of cells having characteristics of dendritic cells was further increased. Cells that were derived from CD34 ุ cell cultures containing 1,25(OH) 2 D 3 had a more potent capacity to present the recall antigen tetanus toxoid to autologous peripheral blood leukocytes and to stimulate a mixed leukocyte response compared to cultures to which 1,25(OH) 2 D 3 had not been added. Our results show that CD34 ุ cells, whose frequency is increased in HNSCC patients, can be differentiated into cells that phenotypically and functionally resemble dendritic cells and that 1,25(OH) 2 D 3 accentuates this differentiation.