The enhancement of tumor metastasis by concurrent inflammatory processes is mainly due to the cytokines TNF and IL-1. In the case of TNF this effect is not restricted to metastasis models as measured by in vivo colony formation but also found in experimental model systems of spontaneous metastasis. Direct effects on the tumor cells or interference with the host NK cell system did not seem to account for the observed TNF effect. Experimental evidence from different test systems rather points to TNF- or IL-1-induced enhanced adhesion of tumor cells to the endothelial cell layer as the underlying mechanism. Blocking of integrin-matrix interactions with monoclonal antibodies or competing peptides inhibited tumor cell adhesion to endothelioma cells in vitro and lung colony formation of tumor cells in vivo.