The induction of acid pathogenesis-related proteins (PR-proteins) of group 1 (PR-1) by systemic virus infections of tobacco plants was investigated during a time period between 3 and 19 days after inoculation. Each leaf position was investigated separately. The PR proteins were detected electrophoretically and, in addition, virus protein was detected by Enzyme Linked Immunosorbent Assay (ELISA). Potato virus X (PVX) and potato virus Y (PVY) were found to highly induce PR proteins in