It has previously been shown that Langerhans cells (LC) accumulate in skin tumours due to production of factor(s) by tumour cells (Halliday et al., Immunology, 77:13, 1992). To determine factors which lead to this accumulation, bioactive TNFα, GM-CSF, and IL-3 were quantitated in a range of tumour lines with differing abilities to accumulate LC. There was no detectable IL-3, and GM-CSF did not correlate with LC. Those tumours which accumulated LC contained significantly lower levels of bioactive TNFα than tumours which did not accumulate LC. Skin grafts or epidermal cell suspensions from above one of the tumours were able to induce protective anti-tumour immunity in naive mice. The tumours with high LC densities were all progressors; those which did not accumulate LC were regressors. Hence we propose that the low level of TNFα produced by some tumour lines is insufficient for LC migration from the tumour, thus inhibiting the induction of anti-tumour immunity. If given the opportunity to migrate to local lymph nodes, these LC are able to provide protective immunity.