To investigate the potential of antibody derivatives to provide passive protection against enteric infections when supplied orally in crude plant extracts, we have expressed a small immune protein (SIP) in plants using two different plant virus vectors based on potato virus X (PVX) and cowpea mosaic virus (CPMV). The epsilonSIP molecule consisted of a single-chain antibody (scFv) specific for the porcine coronavirus transmissible gastroenteritis virus (TGEV) linked to the epsilon-CH4 domain from human immunoglobulin E (IgE). In some constructs, the sequence encoding the epsilonSIP molecule was flanked by the leader peptide from the original murine antibody at its N-terminus and an endoplasmic reticulum retention signal (HDEL) at its C-terminus to allow the expressed protein to be directed to, and retained within, the endoplasmic reticulum. Western blot analysis of samples from Nicotiana clevelandii or cowpea tissue infected with constructs revealed the presence of SIP molecules which retained their ability to dimerize. The analysis of crude plant extracts revealed that the plant-expressed epsilonSIP molecules could bind to and neutralize TGEV in tissue culture, the levels of binding and neutralization reflecting the level of expression. Oral administration of crude extracts from SIP-expressing plant tissue to 2-day-old piglets demonstrated that the extracts which showed the highest levels of in vitro neutralization could also provide in vivo protection against challenge with TGEV.