Background: Polysaccharides isolated from medicinal mushrooms have been shown to exhibit antitumour bioactivity, but the quality and quantity of these polysaccharides are affected by variations in medium composition and operational conditions in the submerged culture. This study was conducted to elucidate quantitatively the effects of sodium chloride (NaCl) supplementation on the production and quality of exopolysaccharides from the medicinal mushroom Phellinus linteus. Results: Supplementation of NaCl to submerged cultures of P. linteus significantly affected the morphology, the exopolysaccharide (PLPS) production, its average molecular weight and its biological activity. Phellinus linteus grew in a filamentous form in NaCl-free medium, but a pellet form became predominant as NaCl supplemention increased. The specific growth rate decreased monotonically from 0.18 to 0.01 day-1 as the NaCl concentration increased from 0.0 to 2.0 g dm-3 (34.22 mmol dm-3); however, the specific product yield increased monotonically from 3.9 to 98.4 mg g-1 (25.2-fold enhancement). Both 14.3-fold enhancement (from 23 to 329mg dm-3) in the production of PLPS and 1.5-fold enhancement (from 670 to 995 pg TNF-α (tumour necrosis factor α) cm-3) in its biological activity were achieved as the NaCl concentration increased from 0.0 to 1.0 g dm-3 (17.11 mmol dm-3). The biological activity of PLPS was highly correlated with its average molecular weight (coefficient R2 = 0.90). Conclusion: The average molecular weight of PLPS was key to its biological activity. Both the quantity and bioactivity of PLPS could be significantly enhanced by appropriate NaCl supplementation to the submerged culture of the medicinal mushroom.