NAKAZAWA, I. and IWAIZUMI, M. A Role of the Cancer Cell Membrane Fluidity in the Cancer Metastases: An ESR Study. Tohoku J. Exp. Med., 1989, 157 (3), 193-198 - AH66F or Yoshida sarcoma (YS) cells were transplanted intraperitoneally into male Donryu rats. Cancer cells obtained from ascites were suspended in saline solution (107cells/ml) after washing. Then, 0.1ml of each suspension obtained from both strains was injected into the tail vein of 5 rats, respectively. Each metastatic nodule, 1mm or less in a diameter, thus obtained was then injected into the peritoneal cavity in which these metastatic cells come to free. After 10 days, cancer cells obtained from each ascites were suspended in phosphate buffered saline (Ca2+ and Mg2+ free, pH 7.2) after washing. Each suspension (107cells/ml) was violently vibrated with a definite amo unt of 5-doxylstearic acid and spin labeling of cancer cell membrane was done. Furthermore, each specimen thus obtained was subjected to the electron spin resonance (ESR) measurement and the order parameter was determined from the spectra. In both YS and AH66F strains, the cell membrane fluidity of the metastatic cancer cell was increased at each temperature measured from 5°C through 35°C. The results obtained here suggest that the change of the cell membrane fluidity of cancer cell is closely related with the cancer metastases.