We examined dendritic cell (DC) status in SCID and RAG2 -/- mice to assess the influence of T cells on DC development and function in vivo. These mice have reduced numbers of DC in the epidermis and lymph nodes draining hapten-sensitized skin. Epidermal DC in these mice were defective in presenting antigen in vivo to adoptively transferred, hapten-sensitized T cells from normal mice. Likewise, draining lymph node DC were deficient in their capacity to stimulate naive T cells in vitro and in vivo. DC numbers as well as the impaired ability to present antigen in vivo, were corrected by reconstituting these animals with normal T lymphocytes, suggesting that T cells are crucial for normal DC maturation and function in vivo.