Abstract Tyrosine kinases are ubiquitous enzymes that have been shown to be involved in many cellular functions, including growth and differentiation. Recent studies have shown that they are also involved in integrin signal transduction pathways. Since integrins are known to be involved in cellular adhesion and thus in invasion and metastasis, the possible involvement of tyrosine kinases in invasion was tested. Tumor cell invasion was measured using filter inserts coated with Matrigel, a substance that closely resembles the natural basement membrane. A highly metastatic subline of BALB/c mammary carcinoma (410.4) cells was shown to invade nearly three times as much as a low metastatic subline (168.1). Genistein, an inhibitor of tyrosine kinases, was found to inhibit invasion of 410.4 cells with an EC 50 of approximately 1 μM. At a concentration of 37 μM, there was almost complete inhibition of invasion by genistein, whereas the structural analog, daidzein, which does not inhibit tyrosine kinases, had only a small effect. At higher concentrations (370 μM), daidzein also caused marked inhibition. Genistein was able to inhibit invasion at concentrations having little effect on cell growth. However, for daidzein, most of the effect on invasion was apparently due to its effect on growth inhibition. The relatively specific effect of genistein to inhibit tumor invasion suggests a role for tyrosine phosphorylation in this process. Genistein or other tyrosine kinase inhibitors may be effective inhibitors of tumor invasion and metastasis.