The fate of ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) was examined in leaves of both resistant and susceptible plants from a Pisum sativum × Pisum fulvum cross after challenge with Erysiphe pisi. High performance liquid chromatography and 1-dimensional gel electrophoresis revealed a 51 kDa protein that was much more abundant in leaves of resistant versus susceptible plants, and which was identified as the Rubisco large chain precursor by mass spectrometry. Greater quantities of Rubisco in resistant tissue suggested that the protein may serve as an important nutrient source for E. pisi in infected susceptible leaves. Liquid chromatography — tandem mass spectrometry revealed three defense-response proteins only in extracts from resistant tissue, while proteins similar to the antimicrobial peptides viscotoxin and phoratoxin from mistletoes, and bubble protein from the yeast Williopsis mrakii, were found only in susceptible tissue. Greater numbers of DNA synthesis and regulation proteins were also identified in the susceptible vs. resistant tissue, probably resulting from changes in host metabolism induced by E. pisi.