Most media for the culture of plant cells contain two or more sources of nitrogen, and these appear to serve in part to balance changes in pH that occur during the growth cycle in batch cultures. Data presented in the present paper indicate that provision of an external pH control (pH-stat) makes it possible to grow cell lines initiated fromIpomoea purpureaandGlycine maxwith ammonium as the only nitrogen source. Furthermore, with ammonium hydroxide as the titrant for pH control, the level of ammonium in the medium can be kept at a selected level (combined pH-stat and ammonium-stat) thus providing a greater degree of control over nutrient concentrations during growth than has been possible heretofore.A culture ofIpomoeacells grown under pH-stat – ammonium-stat conditions was comparable in growth rate and other parameters with one grown in a medium containing both ammonium and nitrate without pH control. However, the controlled culture produced more cells per milliliter than the one without control, and these cells remained high in nitrogen (milligram per gram basis) while the nitrogen content of cells in the culture without control declined late in the growthperiod.