The current disclosure provides methods for detecting and quantitating the 6-O-methylguanine-DNA methyltransferase protein (MGMT) directly in biological samples that have been fixed in formalin by the method of Selected Reaction Monitoring/Multiple Reaction Monitoring (SRM/MRM) mass spectrometry. Such biological samples are chemically preserved and fixed with formaldehyde containing agents/fixatives and may include formalin-fixed tissue/cells, formalin-fixed/paraffin embedded (FFPE) tissue/cells, FFPE tissue blocks and cells from those blocks, and tissue culture cells that have been formalin fixed and/or paraffin embedded. A protein sample is prepared from the biological sample and the MGMT protein is quantitated in the sample using SRM/MRM mass spectrometry by quantitating one or more fragment peptides.