Production of useful peptides using recombinant plant cells is often limited by the stability of the newly translated peptides in the plant expression system. As an initial step toward producing peptides in transformed plant cell cultures, we examined the stability of exogenously added arginine vasopressin (AVP) peptide in a suspension culture of Nicotiana plumbaginifolia cells. The peptide was lost rapidly from the plant cell culture, but the rate of loss was slowed significantly by the addition of the peptide, bacitracin, at a concentration of 150 mu g/mL. At higher concentrations, bacitracin substantially inhibited the growth of the plant cells in culture.