Dendritic cells (DC) are powerful antigen presenting cells, which have the unique capacity to stimulate naive T cells. In spite of the well-known decline of T cell function in old age, little information is available on whether DC are also affected by the aging process. This is mainly due to problems with the isolation and purification of DC. Rapid progress in the characterization of DC has been made in recent years, as simple methods to generate large numbers of DC from precursors have been developed. It was the aim of the present study to compare monocyte derived DC from old and young healthy persons. The generation of DC from blood monocytes in response to GM-CSF and IL-4 treatment was similar in cells from young and old persons. The DC population thus obtained had a typical dendritic morphology and expressed DC surface markers, such as HLA class II, CD1a, CD11c, CD54, CD80 and CD86, but not CD14 for a period of up to three weeks in culture. DC from young and old persons produced IL-12 and TNF-alpha and responded equally well to maturation-inducing stimuli. DC maturation was stimulated by purified protein derivative (PPD) of Mycobacterium tuberculosis, whole inactivated influenza virus and by influenza split vaccine, but not by purified viral RNA. When tested for their antigen-presenting capacity, DC from young and old persons were capable of stimulating the proliferation and the cytokine production of T cells. It was of particular interest that CD45RA(+) as well as CD45RO(+) T cells from aged donors were unable to respond to stimulation with influenza proteins presented by monocytes, but were triggered to proliferate and to produce cytokines when antigen was presented by DC. The results demonstrate that DC from old persons (a) may still function as powerful antigen-presenting cells provided the right differentiation and maturation stimuli are present; (b) are capable of mobilizing residual capacity in senescent T cells and (c) may therefore represent a potent tool for immunotherapy and vaccines in old age.