The motile behavior of two tumor cell variants of the murine BW 5147 lymphosarcoma line, displaying different metastatic capabilities, was analyzed. When placed on top of a confluent monocellular layer of fibroblastic cells, the nonmetastatic lymphoma cells did not carry out any appreciable translocation or shape modification, whereas the metastatic cells displayed intense pseudopodal activity and performed positional shifts. Both these aspects of cell motility were approached through quantitative assays, demonstrating a highly significant difference between the two variant lines. In addition, the metastatic cells were shown to penetrate underneath the fibroblastic monolayer, whereas the nonmetastatic cells were unable to invade. We suggest that the difference in motile behavior is at the basis of the different invasive potencies of the variants. Since in vitro monolayer invasion assays mimic the extravasation of blood-borne cells, we further speculate that in this particular model system, cell motility is the discriminating property that determines whether disseminated tumor formation will occur after intravenous injection of either cell line.