A method for estimating plant cell growth in suspension cultures was developed on the basis of changes in osmotic pressure of the medium. During cultivation of Nicotiana tabacum and Wasabia japonica cells in Murashige-Skoog (MS) medium using sucrose as the carbon source, sucrose was hydrolyzed to glucose and fructose by invertase with a resultant increase in both the molarity and osmotic pressure of the medium. Under this condition, there was no linear relationship between the increase in cell concentration and the changes in osmotic pressure. When glucose was used as the carbon source, the increase in osmotic pressure of the medium was not observed. A linear relationship between cell concentration increase (Delta X [g/l]) and osmotic pressure decrease (Delta P [atm]) was obtained during cultivation of N. tabacum and W. japonica. In suspension cultures of N. tabacum, W. japonica, Catharanthus roseus and Oryza sativa cells, the dry cell concentrations calculated from the osmotic pressure changes coincided well with those obtained by the gravimetric method.