The identification of isoforms is one of the great challenges in proteomics due to the large number of identical amino acids preventing their separations by two-dimensional electrophoresis. Matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOFMS) has become a rapid and sensitive tool in proteomics, notably with the new instrumental improvements. In this study, we used several acquisition modes of MALDI-TOFMS to identify isoforms of porcine glutathiones S-transferase. The use of multiple proteases coupled to the different acquisition modes of MALDI-TOFMS (linear, reflectron, post-source decay (PSD) and in-source decay, positive and negative modes) allowed the identification of two sequences. Moreover, a third sequence is pointed out from a PSD study of a tryptic ion revealing the modification of the amino acid tyrosine 146 to phenylalanine.