The heavy chain monoclonal antibody (HC MAb) was produced in suspension cultures of genetically modified Nicotiana tabacum. The HC MAb secreted to the medium was unstable due to unfavorable interactions in the plant cell medium. The addition of gelatin (5g/l) stabilized the extracellular HC MAb and increased its production 10-fold. A kinetic model was developed describing the interaction between the secretedprotein and the stabilizer. The model accounted for the inactivation of the protein by simple aggregation and general instability. It was assumed that the secreted protein and the stabilizer form a stable complex. Culturing the cells semicontinuously could further increase the productivity of HC MAb.