An in vitro system was developed to induce and identify Xylella fastidiosa proteins that were differentially expressed in the presence of callus-derived extracts from its host, the citrus cultivar Pera. To optimize the induction, we first developed a single culture medium for the growth of both, host and bacteria. This medium, CPXPm7, which mimics the citrus xylem sap, showed that X. fastidiosa at 72 h post-incubation had 101 colony forming units mL(-1), while Pera cells had the highest fresh weight content (0.79 g). After testing various methods of co-cultivation of the bacteria and host callus grown in this single medium, the best induction procedure was to grow X. fastidiosa in a solid medium amended with an extract of Pera callus grown in CPXPm7. Analysis, by two-dimensional electrophoresis, of the X. fastidiosa proteins (120 mu g of total proteins) grown in the presence of Pera callus extract revealed 414 differentially expressed protein spots when compared to the protein profile obtained in the absence of the extract. The system developed in this study improves the induction and analysis of differentially expressed proteins of X. fastidiosa, which may be involved in pathogenicity.