We investigated the mechanism of the enhancement of metastatic potential induced by transfection of the fyn gene, a member of the src family. We employed two murine fyn cDNA-transfected clones, ML-SN1 and ML-SN2, which were previously established from an ML-01 low-metastatic clone of Meth A sarcoma of BALB / c mice and were proven to have higher metastatic ability than ML-01 and the mock-transfected clone ML-MT-neo (Takayama et al., 1993). Our present investigation revealed that the two transfectants showed higher metastatic ability and higher rates of adherence to type IV collagen than ML-MT-neo. However, no difference was found in in vitro or in vivo growth rates, attachment to laminin or endothelial cells or cell motility through a reconstituted basement membrane. Analysis of surface membrane proteins labeled with (125)I on SDS-PAGE showed that a 29 kD band specifically bound to type IV collagen-coupled beads was more intense in ML-SN2 than in ML-MT-neo. Genistein, a protein tyrosine kinase inhibitor, dramatically reduced protein tyrosine kinase (PTK) activity of ML-SN2 in a dose-dependent fashion, corresponding to the reduction of adhesiveness to type IV collagen. The expression of the type IV collagen-binding protein (p29) of ML-SN2 was also reduced significantly by genistein treatment. These results suggested that the fyn product in Meth A cells augments the expression of a type IV collagen-binding protein through elevation of the PTK activity of the membrane fraction and thus facilitates the metastasis of Meth A.