A plastid transformation vector for a stably transforming a plastid genome is provided. The vector includes, as operably-linked components, a first flanking sequence, a DNA sequence coding for a therapeutic human IFN, which is capable of expression in the plastid and a second flanking sequence. The invention also provides isolated and purified IFN, wherein the IFN is configured in a monomeric or multimeric form and is a structural equivalent to orally administered human IFN. Also provided are methods for variable-expressing biopharmaceutical proteins in plants suitable for mammal consumption. The method includes integrating a plastid transformation vector into a plastid genome of a plant cell; growing the plant cell to express a biopharmaceutical protein, such as therapeutic human interferon IFN. Also disclosed are plants transformed with the aforementioned vectors, and the progeny thereof. Also, disclosed is the IFN, which is IFN.alpha.2b.