The effects of culture pH on the production, molecular weight distribution and the bioactivity of polysaccharides produced by Agaricus blazei were evaluated by four pH-controlled batch cultures. As the culture pH of each batch was controlled from 4.0 to 7.0, the maximum polysaccharide concentration increased from 561 to 1252 mg dm(-3), but the average molecular weight of the polysaccharides decreased monotonically from 1080 kDa to 600 kDa, the relative amount of beta-glucan from 56% to 33%, and the TNF-alpha release by macrophage cells from 1440 to 760 pg per 5 x 10(4) cells. Polysaccharides with higher molecular weights, beta-glucan contents and bioactivities were obtained in the submerged cultures of Agaricus blazei at lower culture pH but with lower yields. (C) 2004 Society of Chemical Industry.