Dendritic cells (DCs) have been shown to enhance anti-tumor immune responses in several preclinical models. Furthermore, DC-like function can be elicited from peripheral blood monocytes cultured in vitro with interleukin-4 and granulocyte-macrophage colony-stimulating factor. For this reason, a phase 1 study was initiated at the Surgery Branch of the National Cancer Institute to test the toxicity and biological activity of the intravenous administration of peripheral blood monocyte-derived DCs. The DCs were generated by 5- to 7-day incubation in interleukin-4 (1,000 U/mL) and granulocyte-macrophage colony-stimulating factor (1,000 U/mL) of peripheral blood monocytes obtained by leukapheresis. Before administration, the DCs were pulsed separately with the HLA-A*0201-associated melanoma epitopes MART-1(27-35) and gp-100-209-2M. The DCs were administered four times at 3-week intervals. A first cohort of patients (n = 3) was treated with 6 x 10(7) DCs and a second cohort (n = 5) with 2 x 10(8) DCs (in either case, one half of the DCs were pulsed with MART-1(27-35) and the other half was pulsed with gp-100-209-2M). In a final cohort under accrual (n = 2) 2 x 10(8) DCs were administered in combination with interleukin-2 (720,000 IU/kg every 8 hours). The recovery of DCs after in vitro culture ranged from 3% to 35% (mean, 15%) of the original peripheral blood monocytes. Administration of DCs caused no symptoms at any of the doses, and the concomitant administration of interleukin-2 did not cause toxicity other than that expected for interleukin-2 alone. Monitoring of patients' cytotoxic T lymphocyte reactivity before and after treatment revealed enhancement of cytotoxic T lymphocyte reactivity only in one of five patients tested. Of seven patients evaluated for response, one had a transient partial response with regression of pulmonary and cutaneous metastases. A relatively large number of DCs can be safely administered intravenously. The poor clinical outcome of this study perhaps could be explained by the type of protocol used for DC maturation, the route of administration, or both. For this reason, this clinical protocol was interrupted prematurely, whereas other strategies for DC preparation and route of administration are being investigated at the authors' institution.