Mass spectrometry is emerging as one of the most promising analytical techniques to examine simultaneously hundreds of analytes quickly, precisely, and accurately, using minute sample volumes. Currently, a major bottleneck in the verification phase of putative biomarkers is the lack of methods/reagents to quantify low levels of analytes in biological fluids. A major objective is to establish a high-throughput multiple reaction monitoring (MRM) assay capable of quantifying low-abundance proteins or peptides in biological fluids (low μg/L range) using mass spectrometry. The experimental procedure we propose, called immuno-mass spectrometry, consists of immuno-capturing analytes of interest from relevant biological fluids in 96-well microtiter plates and performing in-well tryptic digestion, with subsequent MRM of digested peptides on a triple quadrupole mass spectrometer. With such a strategy, limits of detection of 0.1-1 μg/L proteins in serum with a coefficient of variation of <20% can be obtained. This methodology could be adapted quickly and easily to potential candidates of interest, thus providing a much needed technology to bridge the gap between discovery and validation platforms.