Effects of various environmental factors on cell size index(FCW/DCW) in Thalictrum rugosum. Lithospermum erythrorhizon and Taxus cuspidata plant cell suspension cultures were investigated. Time course change of cell size index were also observed. In batch cultures, FCW/DCW increased according to the decrease of sugar concentration. For short-term experiment within 24 hr, FCW/DCW value could be reduced significantly by increasing sugar concentration. When an osmoticum such as mannitol was added, FCW/DCW converged to a low value. Therefore, it was confirmed that osmolality of the medium was important in determining cell size or water content of the cells. Inorganic salts or treatment with organic solvent also exhibited some effect on the cell size index. However, pH and centrifugal force did not show any influences. On the other hand, it was found that the addition of Pluronic F-68 reduced FCW/DCW. By combining these results effectively, it may be possible to increase the cell concentration in high density culture to a higher extent.