Langerhans' cells migrating from contact-sensitized skin were found to up-regulate expression of macrophage-derived chemokine (MDC) during maturation into lymph node dendritic cells (DCs). Naı̈ve T cells did not migrate toward MDC, but antigen-specific T cells rapidly acquired MDC responsiveness in vivo after a subcutaneous injection of antigen. In chemotaxis assays, maturing DCs attracted activated T cells more strongly than naı̈ve T cells. These studies identified chemokine up-regulation as part of the Langerhans' cell maturation program to immunogenic T cell–zone DC. Preferential recruitment of activated T cells may be a mechanism used by maturing DCs to promote encounters with antigen-specific T cells.