ABSTRACT. Differentiating cells of Dictyostelium discoideum were spinlabeled with 5-nitroxide stearic acid or phosphatidylcholine spin labels, and the electron spin resonance spectra of the cells were measured. There was no difference in the flexibility of the lipid bilayer in the membranes between preaggregation cells and disaggregated slug cells. The temperature dependence of the overall splitting value for 5-nitroxide stearic acid was also indistinguishable for the two cell types in the range of 4°C to 25°C. Two distinct inflection points were observed at 15°C and 19.5°C in the temperature dependence, suggesting the presence of a phase transition in the lipid bilayer portion of the membranes. We thus concluded that there is no change in the membrane fluidity of the cells on formation of a multicellular organization. In addition, trypsinization of slug cells, which enhances Con A-induced redistribution of the Con A-binding sites on the cell surface (Kawai, in preparation), did not cause any change in the flexibility of the lipid bilayer. This suggests that the increased mobility of the sites has no apparent correlation to the bulk membrane fluidity.The lateral mobility of such receptor sites as the antigen and lectin binding sites intercalated in the plasma membranes has been considered to be under the control of the fluid state of the lipid bilayer of the membranes (13). On this basis, it has been widely supposed that the difference in the lateral mobility of the sites in the cell surface between normal and tumor or transformed cells is due to possible changes in membrane fluidity (4, 7, 11). Thus, we conducted, spin-labeling studies to examine whether there is any difference in the fluidity of the lipid bilayer of the membranes (5, 23) of these cells.In the development of the cellular slime mold Dictyosteliurn discoideum, amebae grow and multiply during the vegetative phase by feeding on bacteria. After the cessation of cell growth, amebae aggregate at a collecting point and form a slug-shaped cell mass. The slug undergoes a series of morphogenetic movements and finally produces a fruiting body consisting of spores and stalk cells. Several changes in the properties of the cell surface have been found during cell differentiation (1,20,22). By examining changes in the reactivity of differentiating cells with concanavalin A(Con A), a lectin which binds to the carbohydrate containing ligands on the cell surface (12), we found that Con A-induced agglutinability of the cells decreased as cells differentiate, especially upon the formation of a multicellular organization (9). We also found that the mobility of Con A-binding sites in the cell surface remarkably decreased during this process (Kawai and Takeuchi, in preparation ABSTRACT. Differentiating cells of Dictyostelium discoideum were spinlabeled with 5-nitroxide stearic acid or phosphatidylcholine spin labels, and the electron spin resonance spectra of the cells were measured. There was no difference in the flexibility of the lipid bilayer in the membranes betw...