In contact with cells, TMA-DPH is instantaneously incorporated into the plasma membrane. TMA-DPH is concentrated in lysosomes and highly acidic late endosomes. The TMA-DPH fluorescence lifetime remains constant, 6.2±0.2 ns, below the same critical concentration of 2 μM, but decreases significantly with higher concentrations. This decrease, however, slows down above 5 μM. The TMA-DPH fluorescence anisotropy displays a similar evolution to the fluorescence lifetime. It is first constant, 0.283±0.003, again until 2 μM, and then decreases rapidly to 0.270±0.003 with 5 μM, continuing to fall, but at a lower rate with higher concentrations.