Fatty acid spin labels were used to measure the inherent flexibility of the lipid acyl chains in intact cell membranes of 3T3 mouse fibroblasts and several varieties of transformed 3T3 cells. No significant differences in inherent lipid flexibility were detected in the normal and transformed mouse fibroblasts. These results are compared with similar results obtained earlier for chick embryo fibroblasts. For the cells studied, a difference in the mobility of membrane glycoproteins of normal and transformed fibroblasts has either been demonstrated directly or inferred from differential agglutinability by other investigations. Therefore, a basis for correlating glycoprotein mobility and lipid motion in these cells was not found.