Total, soluble and microsomal fraction proteins were isolated from leaves of healthy and virus-infected cucumber and tomato plants, and separated by SDS-polyacrylamide gel electrophoresis. The analysis revealed the accumulation of a microsomal fraction protein in severely infected plants: cucumber with zucchini yellow mosaic virus (ZYMV) and tomato with cucumber mosaic virus (CMV). The proteins accumulated (MP 39) in the hosts when they showed severe disease symptoms exhibited the same electrophoretic mobility and had a molecular mass of approximately 39 kDa. The MP 39 was not revealed by concanavalin-A, and was present in the aqueous phase from Triton X-114 fractionation, indicating that the MP 39 is a peripheral membrane protein. Bidimensional electrophoretic analysis revealed the same characteristics for the protein accumulated in infected cucumber and tomato. Antibodies directly raised against virus particles failed to react with the MP 39, suggesting the accumulated protein is unrelated to any of the structural proteins of ZYMV and CMV, being probably a host encoded protein that accumulates in severely virus-infected plants. While activities of soluble and cell wall bound peroxidases were increased in infected plants, no peroxidase activity was found associated with MP 39. The data obtained in this work are a good base for subsequent studies on the role of the MP 39 in infected plants.334453461