To permit simultaneous long-term tracing of mixed-cell populations using different fluorescent colors, we have developed a probe whose applications should be similar to those of CFDA SE (commonly called CFSE). Cells labeled with our SNARF-1 carboxylic acid, acetate, succinimidyl ester have orange-red fluorescence that can be easily distinguished from that of cells loaded with green-fluorescent tracers such as CFDA SE.