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Spin-labelled fatty acids I(12,3) and I(1,14) were incorporated into microsomal membrane of cryptorchid mouse testis and Leydig cell tumor as well as liver. The freedom of motion of spin of I(12,3) was more restricted in testis microsome than in liver. At the lower temperatures, the freedom of motion of spin in the tumor microsomes was similar to that in the testis, but at higher temperature (20-50 degrees C) was much greater. Plotting of the empirical parameter, h0/h-1, calculated by the spectra of I(1,14), against the reciprocal of the absolute temperature clearly showed two inflection points in both liver and testis microsomes, one at 19 decrees C and the other at 30 degrees C. On the other hand, tumor microsomes lacked these break points and permitted spin to move more freely. These results suggest that tumor microsomes contain the increased fluidity. The importance of membrane fluidity in relation to steroid biosynthesis was also discussed.