This chapter illustrates the identification of N-terminal myristyl blocking groups in proteins. The catalytic subunit of cyclic-AMP-dependent protein kinase (C subunit) and the regulatory subunit of protein phosphatase 2B contain myristic acid covalently attached to their amino-terminus. The discovery of myristic acid at the N-terminus of a protein phosphatase and a protein kinase suggests that this unusual blocking group may be involved in the interaction of converter enzymes with their protein substrates. The myristyl group could also play a role in maintaining the subunit-subunit interactions between the regulatory (B) and catalytic (A) subunits of calcineurin and between the regulatory and catalytic subunits of cyclic-AMP-dependent protein kinase. The myristyl group may be possibly involved in the translocation of the C subunit from the cytoplasm to the nucleus, across the nuclear membrane. Fast atom bombardment (FAB) mass spectrometry is considered the most useful mass spectrometric technique for identification of myristic acid in blocked peptides. Involatile and thermally labile compounds can be studied by this method and very small amounts of material can be used.