Cell motility is an important factor in the metastatic process that can be affected by environmental conditions. A quantitative study was made of the relationship between cell motility and the colonization potential of a mouse colon adenocarcinoma cell line (MCA-38). MCA-38 cells grown in culture did not produce hepatic or pulmonary colonies following ileocolic or tail vein injection, respectively. In contrast, MCA-38 cells adapted to grow in the mouse produced colonies in both organs. The motility of the MCA-38 cells that did not produce colonies, as determined by the depth of penetration into cellulose nitrate filters (8 micron pore size), was significantly less than that of MCA-38 cells with colony-forming potential. Return to in vitro growth resulted in both a loss of colonization potential and a reduction in motility. In this system, secondary organ colonization and in vitro cell motility are positively correlated, suggesting an association between cell motility and metastatic potential.