Lewis lung tumor cells from tumors of both high and low metastatic potential were isolated from the leg muscle of C57-B1 mice and were purified by density gradient centrifugation. The purified cells retained most of their tumor-forming capacity; however, the difference in the metastatic capacity of the 2 cell lines increased with the purification process. The purified and viable cells were subjected to different biophysical studies. Electron spin resonance studies showed that the motional freedom of lipid and protein molecules of the cellular membrane is higher in the low metastatic cell population. Both cell types reduce penetrating and non-penetrating free radicals, but the reducing ability of the low metastatic cell population is 4 times greater than that of the high metastatic cell line. These findings argue for increased membrane dynamics in the low metastatic cell population.