In situ staining techniques were used to visualize physical interactions between dendritic cell subsets and naive Ag-specific CD4 T cells in the lymph node. Before injection of Ag, CD8+ dendritic cells and naive OVA-specific CD4 T cells were uniformly distributed throughout the T cell-rich paracortex, whereas CD11b+ dendritic cells were located mainly in the outer edges of the paracortex near the B cell-rich follicles. Many OVA-specific CD4 T cells were in contact with CD8+ dendritic cells in the absence of OVA. Within 24 h after s.c. injection of soluble OVA, the OVA-specific CD4 T cells redistributed to the outer paracortex and interacted with CD11b+, but not CD8+ dendritic cells. This behavior correlated with the uptake of OVA and the presence of peptide-MHC complexes on the surface of CD11b+ dendritic cells, and subsequent IL-2 production by the Ag-specific CD4 T cells. These results are consistent with the possibility that CD11b+ dendritic cells play a central role in the activation of CD4 T cells in response to s.c. Ag.