We introduce a new multistep mass tagging technique and show its utility for reducing sample complexity when coupled with two-dimensional liquid chromatography/nano-electrospray ionization ion trap mass spectrometry (2D LC/nano ESI-MS). Solid-phase mass tagging reagents were used to identify and obtain relative quantitation of membrane proteins from two established breast cancer cell lines, BT474 and MCF7. The results presented in this study show that sample complexity can be reduced with corresponding increases in protein identification and quantitation.