Pokeweed antiviral proteins (PAPs) become novel therapeutic agents in relation to application in human viral diseases and cancer, as well as potent tools in plant system for defending viral infection. We have studied the expression characteristics of PAPs in pokeweed plants by western blot analysis. PAP-I was constitutively expressed in leaves, stems and roots of the pokeweed plant, while PAP-II was not expressed in roots. The expression of PAP-II began in May and then gradually increased with development of the plants. The PAP-II expression was induced and/or stimulated not only by biotic stresses, such as insect pests and viral infection, but also by abiotic stresses, like drought. Interestingly, low-light intensity was found to be more effective than high-light in the expression of both PAP-I and PAP-II. Our results suggest that PAP-II appears to have an additive effect in terms of protection of the plant against pathogens during summer-time when the plant actively grows and is attacked by various pathogens.