Acidification of cytosolic pH can cause the redistribution of endocytic structures. We therefore measured cytosolic pH using the fluorescent probe BCECF in each of the cell types after incubation for 8 h at either the permissive (34°C) or nonpermissive (40°C) temperatures. For each condition, a standard curve was generated to calculate both the pK_a of the dye and the intracellular pH. The mutant cells were only slightly acidified. Values are from one experiment wherein a standard curve was prepared for each condition. Similar results were obtained for wt cells and ldlF cells at 40°C in several other experiments.