Male Donryu rats weighing from 100-120 g were transplanted with AH100B, AH66F or Yoshida sarcoma (YS) [hepatoma] cells i.p. After collecting ascites, while not bloody, the cancer cells were washed 3 times with Dulbecco''s phosphate buffered saline (PBS) without Ca2+ and Mg2+ (pH 7.2). Each sediment was suspended in PBS. Each suspension in which 10 million cancer cells were contained was violently vibrated with 5-doxyl stearic acid, and the cancer cell membrane was spin labeled. ESR spectra were measured for each specimen to obtain an order parameter. The order parameter of AH100B was the highest and that of YS the lowest at each temperature measured from 5-35.degree. C. YS cells have been found in the bloodstream within 24 h, AH66F cells in 1 day and AH100B cells in .ltoreq. 10 days after i.v. inoculation. The accelerated fluidity of the cancer cell membrane may be related to cancer metastasis.