Granulocyte macrophage colony stimulating factor (GM-CSF) is a powerful cytokine with multiple actions. We investigated the effects of low dose daily rhGM-CSF application on monocytes and peripheral circulating dendritic cells (DC) in malignant melanoma patients in vivo. Twenty patients were included; rhGM-CSF was given as daily subcutaneous injections for 14 days. A significant increase was noted in monocytes and granulocytes, starting on the 5th day. Expression of CD95 (Apo-1/Fas) and CD45RO on monocytes increased significantly on the 5th day, and CD4 expression on monocytes increased significantly on the 14th day. Peripheral circulating dendritic cells which were 0.94% in the beginning, increased to 1.35% (P < 0.04) and to 1.96% (P < 0.001) on days 5 and 14, respectively. (C) 2003 Elsevier Science Ltd. All rights reserved.