Objective quantitation of the c-Src protein directly in cancer patient tissue can aid in determining the aggressiveness of an individual patient's tumor as well as help make more informed decisions about choice of therapy. However, the c-Src protein is currently analyzed directly in formalin fixed patient tissue only by immunohistochemistry methodology which is at best subjectively semi-quantitative. This invention describes an objective quantitative assay for the c-Src protein using mass spectrometry as the analytical methodology. Specific peptides, experimentally discovered characteristics about the peptides, and experimentally established assay conditions based on those peptide characteristics are provided for use in a mass spectrometry-based Selected Reaction Monitoring (SRM) assay in order to measure relative or absolute quantitative levels of c-Src directly in a protein preparation obtained from a formalin fixed cancer patient tissue sample.