Cholesterol affects lipid mixing. Lowering pH initiated diffusion of lipid dye from the RBC to the HAS cell membrane. (A) Changes in fluorescence intensity (F) were measured in a small ROI at the periphery of HAS cells opposite to site of the bound of RBC using a fixed area (896 pixel). Cholesterol enrichment led to faster lipid dye transfer (i) compared with untreated cells (ii). In i and ii time zero is the onset of dye diffusion and iii is the F measurement of an unfused cell pair. (B) Time required to reach half-maximum fluorescence (n > 5).