Sequences encoding the immunoglobulin heavy-chain variable (VH) domains were engineered in a new general purpose vector to transform plants via Agrobacterium.  The expression of an isolated VH domain (IVD) after introduction into the plant genome has been monitored by northern, western and immuno-histochemical analysis.  Immunoblotting showed that the polypeptide was stably expressed and accounted for up to 1% of the soluble protein fraction.  It is therefore proposed that single immunoglobulin domains of suitable specificity expressed in plants may constitute an effective system to inhibit the activity of molecules involved in plant pathology or plant development.