Understanding the complex relationships between genomics, transcriptomics, and proteomics requires the development of more sensitive and rapid methods of multiplexed protein analysis. This is necessary to understand the relationship between cellular responses to environmental stresses, disease progression, and/or drug treatment; however, most methods are limited by low sensitivity, nonspecificity, and minimal multiplexing capacity. To more fully explore the relationship between multiple cellular pathways, we have developed a novel antibody-based multiplex assay using inductively coupled plasma mass spectrometry (ICP-MS), which we term metal-assisted protein quantitation (MAPq). MAPq utilizes lanthanide-conjugated antibodies to simultaneously quantify up to 35 proteins with low pg/mL sensitivity. This method is especially advantageous for low-abundance proteins, a significant limitation of many multiplex MS methods. We observed a limit of detection of 0.5 pg/mL and a limit of quantitation of 5 pg/mL with virtually no background signal. We applied this method to both cultured cells and mouse tissues to investigate changes in low-abundance nuclear and cytoplasmic proteins following drug or environmental stresses. MAPq was found to be at least 10 times more sensitive than Western blots and could detect quantitative changes in protein expression not readily observed using conventional approaches.