Pluronic F-68, PEG 8000, or PEG 20 000 added to cell suspension cultures of transgenic Nicotiana tabacum promoted cell growth and the production of the recombinant murine granulocyte macrophage-colony stimulating factor (mGM-CSF) in a 5-1 stirred tank bioreactor. The specific growth rates were enhanced from 0.27 d(-1) to 0.47 d(-1), 0.37 d(-1) and 0.4 d(-1) when Pluronic F-68, PEG 8000, or PEG 20 000 was added, respectively. The maximum cell density was also increased most to 13.6 g l(-1) when Pluronic F-68 was added (11.3 g l(-1) in the control culture). In terms of mGM-CSF production, PEG 8000 gave the greatest stimulation and with 2 g PEG 8000 l(-1), mGM-CSF increased from 1.6 to 6.6 ng ml(-1).