Interleukin-4 (IL-4) is a pleiotropic cytokine that plays a key regulatory role in the immune system. Recombinant human IL-4 (rhIL-4) offers great potential for the treatment of cancer, viral and autoimmune diseases. Unfortunately, the high production cost of IL-4 associated with conventional expression systems has, until now, limited broader clinical testing, particularly with regard to the more convenient and safer oral delivery of IL-4 as opposed to parenteral injection in patients. In this study, we investigated the feasibility of transgenic plants for the cost-effective production of rhIL-4. IL-4 expression vectors with different modifications under the control of a constitutive cauliflower mosaic virus 35S (CaMV 35S) promoter were introduced into tobacco by Agrobacterium-mediated transformation. Transgenic tobaccos expressing various levels of rhIL-4 protein were generated. Higher expression was achieved through IL-4 retention in the endoplasmic reticulum (ER), with the maximal accumulation being approximately 0.1% of total soluble protein (TSP) in the leaves. No improvement in expression was further achieved by replacing the native signal peptide of IL-4 with the plant signal peptide. The best rhIL-4-expressing vector shown in tobacco was selected and further transferred into potato plants. The analysis of transgenic tubers also revealed various levels of rhIL-4, with the highest being 0.08% of TSP. Sensitive in vitro T-cell proliferation assays showed that plant-derived rhIL-4 retained full biological activity. These results suggest that plants can be used to produce biologically active rhIL-4 and probably many other mammalian proteins of medical significance. Moreover, the production of plants expressing rhIL-4 will enable the testing of plant rhIL-4 by oral delivery for the treatment of clinical diseases.