The capacity to migrate from peripheral tissues, where antigen is encountered, to lymphoid organs, where the primary immune response is initiated, is crucial to the immunogenic function of dendritic cells (DC). The skin is a suitable tissue to study migration. DC were observed to gather in distinct nonrandom arrays ("cords") in the dermis upon culture of murine whole skin explants. It is assumed that cords represent lymphatic vessels. Using a similar organ culture model with human split-thickness skin explants, we investigated migration pathways in human skin. We made the following observations. 1) Spontaneous emigration of Langerhans cells took place in skin cultured for 1-3 d. Nonrandom distribution patterns of strongly major histocompatibility complex class II-expressing DC (cords) occurred in cultured dermis. A variable, yet high (>50%) percentage of these DC coexpressed the Birbeck granule-associated antigen "Lag." Ultrastructurally, the cells corresponded to mature DC. 2) Electron microscopy proved that the dermal structures harboring the accumulations of DC (i.e., cords) were typical lymph vessels. Moreover, markers for blood endothelia (monoclonal antibody PAL-E, Factor VIII-related antigen) and markers for cords (strong major histocompatibility complex class II expression on nonrandomly arranged, hairy-appearing cells) were expressed in a mutually exclusive pattern. 3) On epidermal sheets we failed to detect gross changes in the levels of expression of adhesion molecules (CD44, CD54/ ICAM-1, E-cadherin) on keratinocytes in the course of the culture period. The reactivity of a part of the DC in the dermal cords with Birbeck granule-specific monoclonal antibody "Lag" suggests that the migratory population is composed of both epidermal Langerhans cells and dermal DC. We conclude that this organ culture model may prove helpful in resolving pathways and mechanisms of DC migration.