Production of therapeutic proteins by transgenic plant cell suspension cultures is an attractive system alternative to the other expression system. However, plant cell cultures have shown low expression level of foreign proteins and decreased cell viability by the changes of culture conditions. Therefore, it is necessary to enhance cell viability during the culture period. In this study, a quantitative analysis technique was designed to measure relative cell viability for plant suspension cells which have cell wall and aggregates. It was found that the programmed cell death of plant cells by apoptosis was essentially linked with the apoptotic pathway of animal cells. Therefore, effects of nicotinamide, 3-aminobenzamide and antioxidants on cell viability and apoptosis were examined in transgenic Nicotiana tabacum cells producing hGM-CSF. With those additives, cell viability could be maintained and apoptosis could be redued. In the result, the extracellular production of hGM-CSF could be enhanced 2.5 fold. It was also found that the supplementation of glutathione and ascorbic acid suppressed both the cold stress-induced decrease in cell viability and the increase of total genomic DNA fragmentation.