Tumor necrosis factor (TNF) causes cell necrosis in vivo by damaging the endothelium of the neovasculature. However, its mechanism of action is not well understood. We hypothesized that TNF affects the tumor microenvironment even before neovascularization occurs, thereby increasing lymphocyte locomotion through the peritumoral matrix, a crucial step in tumor cell killing. The effect of TNF on lymphocytes was tested with the type I rat-tail collagen mini-assay in peripheral blood lymphocytes (PBL) from normal donors, a non-migratory PBL cell line (HPB), and a C3H mice splenic lymphocytes. Melanoma cell line (k1735p) was treated with TNFalpha/TNFbeta 10 or 20 pg/microl. The syngeneic splenic lymphocytes were layered on top of the collagen, and their migration into the collagen towards the tumor cells was assessed. Tumor cell viability was evaluated before and after TNF treatment. Paired two-tailed Student's t-test was used for statistical analysis. TNFalpha and TNFbeta had no significant direct effect on locomotion of PBL or HPB. Lymphocyte locomotion was inhibited in the presence of untreated melanoma cells in 7 of 9 assays (statistically significant in four), and it was significantly increased towards TNFalpha- or beta-treated melanoma cells, compared to untreated condition, in 7 of 9 assays (p=0.05 to p=0.0001). The number of viable tumor cells was not significantly different before and after treatment. In conclusion, treatment of tumor cells with TNFalpha or TNFbeta significantly enhances lymphocyte locomotion through the matrix. The effect of TNF is not the result of a direct influence on the lymphocytes, and is not associated with a decrease in the number of viable tumor cells. These findings suggest that TNF interaction with the cell microenvironment induces a change in lymphocyte locomotion.