Whole cells of the eukaryotic microorganism Dictyostelium discoideum have been assayed with membrane fluorescence polarization probes. With diphenylhexatriene (DPH) the steady-state fluorescence polarization remained stable during growth, but significant rise was found as a function of time of development. This variation, discussed as being specific to the plasma membrane, is the first report of a biophysical change occurring at this level during cellular aggregation of D. discoideum. In contrast, with trimethylammonium (TMA)-DPH no change was observed during development. The differences obtained with the two probes are discussed in the light of complementary fluorescence anisotropy decay measurements.
