Previous studies have shown that following skin sensitization there is an accumulation of dendritic cells (DC) in lymph nodes draining the site of exposure. A significant number of the DC which arrive in the lymph nodes bear high levels of antigen, and the available evidence indicates that they are derived from epidermal Langerhans' cells (LC). Although freshly isolated LC are relatively inefficient antigen-presenting cells, the antigen-bearing DC which are found within draining nodes following skin sensitization are highly immunostimulatory. Recent investigations indicate that the functional maturation of LC as they migrate from the skin is reflected by an enhanced capacity to form stable clusters with lymphocytes, and is associated with an increased expression of membrane major histocompatibility complex (MHC) class II (Ia) antigen. By analogy with in vitro studies of LC maturation, it is possible that such changes are effected by granulocyte/macrophage colony-stimulating factor (GM-CSF) and interleukin-1 (IL-1), both of which are products of epidermal cells. The question remains as to the nature of the stimulus that initiates LC migration. In the present study we have examined in mice the effects of intradermal injection of tumour necrosis factor-alpha (TNF-alpha), another epidermal cytokine, on the accumulation of DC in lymph nodes. Murine recombinant TNF-alpha was found to cause a concentration- and time-dependent increase in the number of DC within draining nodes. Under the same conditions of exposure murine recombinant GM-CSF was without effect. Heat treatment of mouse TNF-alpha resulted in an equivalent inhibition of both DC accumulation and cytotoxic activity measured by in vitro bioassay. An interesting observation was that equal concentrations of human TNF-alpha, of equivalent specific activity, failed to influence the frequency of lymph node DC. These data demonstrate that TNF-alpha induces DC accumulation in draining lymph nodes, and we propose that this cytokine may provide one stimulus for LC migration during cutaneous immune responses.