Abstract: ⁹⁹Tc^m-tetrofosmin is a widely available and conveniently prepared tracer that has been shown to be a transport substrate for P-glycoprntein and multidrug resistance protein in vitro and in vivo. Its properties are similar but not identical to those of ⁹⁹Tc^m-sestamibi. The available data suggest that clinical studies involving imaging of muhidrug resisttanee function and in vivo modulation of muhidrug resisttance function could be performed with ⁹⁹Tc^m-tetrofosmin or ⁹⁹Tc^m-sestamibi, but the two should probably not be used interchangeably.