DiI (DiIC18(3); 1,1′-dioctadecyl-3,3,3′,3′-tetramethylindocarbocyanine) uniformly dissolved in vegetable oil in a formulation optimized for membrane labeling or cell tracking. Carbocyanine dyes in oil have been widely used for fate mapping, cell lineage studies, pre-labeling cells for transplantation, and neuronal tracing and labeling.