Peptides from the Insulin-Like Growth Factor 1 Receptor (IGF-1R) protein are provided that are particularly advantageous for quantifying the IGF-1 R protein directly in biological samples, such as samples fixed in formalin. The ionization characteristics of the peptides also are disclosed. The peptides may be used in Selected Reaction Monitoring (SRM) mass spectrometry methods, also referred to Multiple Reaction Monitoring (MRM) mass spectrometry methods. The samples are chemically preserved and fixed, such as tissues and cells treated with formaldehyde containing agents/fixatives including 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 may be prepared from the biological sample and the IGF-IR protein is quantitated by the method of SRM/MRM mass spectrometry by quantitating one or more of the described peptides. These peptides can be quantitated in either modified or unmodified form. An example of a modified form of an IGF-1 R peptide is phosphorylation of a tyrosine, threonine, serine, and/or other amino acid residues within the peptide sequence.