Technical Abstract: SDS-PAGE and 35S-radiolabeling methods previously have been used to study the protein composition of phloem exudates from various plant species. These studies have been successful in identifying numerous proteins ranging in mass from 14 to 98 kDa. However, since soluble proteins account for less than 2% of solutes in phloem sap, a more sensitive technique is required for the study of low abundance and low molecular weight proteins. In this study, we characterized the protein composition of lupin and pea phloem exudates by matrix assisted laser desorption ionization time-of-flight mass spectrometry (MALDI-TOF-MS). Phloem sap was collected from lupin pods (using shallow incisions) or pea seedlings (by placing cut stems in an EGTA solution) with or without the addition of protease inhibitors. The use of either a basic (2-amino-4-methyl-5-nitropyridine + trifluoroacetic acid) or acidic (sinapinic acid) matrix yielded similar MALDI-TOF-MS results and revealed an array of LMW proteins ranging from 2 to 10 kDa. The advantage of MALDI-TOF-MS for studying complex protein mixtures such as phloem and its application to the study of LMW peptide mediated shoot-root communication will be discussed. This research was funded by USDA-ARS Cooperative Agreement No. 58-6250-1-003 and by USDA-CSRS Grant No. 94- 37100-0823 to MAG.