The continuous production of murine granulocyte-macrophage colony-stimulating factor (GM-CSF) by recombinant yeast cells immobilized in a fibrous-bed bioreactor was studied. A high cell density of similar to 68 g/L and a GM-CSF productivity of similar to 3.5 mg/ L . h were attained in the fibrous-bed bioreactor fed with a rich (nonselective, pH 6.7) medium at a dilution rate of 0.16 h(-1). The GM-CSF production was stable even though the fraction of plasmid-carrying cells in the reactor effluent gradually dropped below 5% over a period of 2 weeks. At the end of that period, the immobilized cells in the fibrous matrix still had a high fraction, similar to 26%, of plasmid-carrying cells. Similar results were obtained with reactors operated at 0.05 h(-1) dilution rate and pH 4.0. Although the GM-CSF production was lower at pH 4, the reactor was stably operated for over 4 weeks without contamination or significant loss of productivity. The stable longterm GM-CSF production from the fibrous-bed bioreactor was attributed to the effect of cell immobilization on plasmid stability. Because GM-CSF production was growth-associated, as was found in batch fermentation with free cells, this stabilization effect cannot be attributed solely to the reduced cell growth in the immobilized cell environment. Plasmid-carrying cells were preferentially retained in the fibrous matrix, perhaps because their abilities to adhere to the fiber surface and to form cell aggregates were higher than those of plasmid-free cells.