CCR7, along with L-selectin and LFA-1, mediates homing of T cells to secondary lymphoid organs via high endothelial venules (HEV). CCR7 has also been implicated in microenvironmental positioning of lymphocytes within secondary lymphoid organs and in return of lymphocytes and dendritic cells to the lymph after passage through nonlymphoid tissues. We have generated mAbs to human CCR7, whose specificities correlate with functional migration of lymphocyte subsets to known CCR7 ligands. We find that CCR7 is expressed on the vast majority of peripheral blood T cells, including most cells that express adhesion molecules (cutaneous lymphocyte Ag α 4 β 7 integrin) required for homing to nonlymphoid tissues. A subset of CD27 (neg) memory CD4 T cells from human peripheral blood is greatly enriched in the CCR7 (neg) population, as well as L-selectin (neg) cells, suggesting that these cells are incapable of homing to secondary lymphoid organs. Accordingly, CD27 (neg) T cells are rare within tonsil, a representative secondary lymphoid organ. All resting T cells within secondary lymphoid organs express high levels of CCR7, but many activated cells lack CCR7. CCR7 loss in activated CD4 cells accompanies CXC chemokine receptor (CXCR) 5 gain, suggesting that the reciprocal expression of these two receptors may contribute to differential positioning of resting vs activated cells within the organ. Lymphocytes isolated from nonlymphoid tissues (such as skin, lung, or intestine) contain many CD27 (neg) cells lacking CCR7. The ratio of CD27 (neg)/CCR7 (neg) cells to CD27 (pos)/CCR7 (pos) cells varies from tissue to tissue, and may correlate with the number of cells actively engaged in Ag recognition within a given tissue.