Echistatin, a low molecular weight, RGD-containing protein isolated from the venom of Echis carinatus, inhibited Lewis lung carcinoma cell (3LL) adhesion to immobilized fibronectin and laminin. The inhibition was specific, noncytotoxic, dose-dependent and fully reversible. Echistatin showed a stronger activity in inhibiting cell adhesion to fibronectin rather than to laminin and it resulted about 3-fold more effective than kistrin, an other ROD-snake venom protein, in inhibiting 3LL cell attachment to both substrates. The ability of echistatin to modulate experimental metastasis formation in vivo was also evaluated. A 20% inhibition of the lung metastasis spread with respect to controls was observed when 3LL cells and echistatin were coinjected i.m. into male C57BL/6NCr1BR mice. When echistatin was administered i.p. 1 mu g/g of body weight/72 h x 4 doses into mice bearing Lewis lung carcinoma, it promoted only a 15% inhibition of tumor growth but inhibited by 45% lung metastasis formation. These results demonstrate that echistatin is able to inhibit metastasis attachment and spreading in experimental system in vivo independently by its effect on the primary tumor.