Suspension cultures of a plant cell line initiated from Ipomoea sp. root tissue were grown in defined medium at controlled pH levels of 4.8, 5.6, 6.4, and 7.1 with ammonium and nitrate as the nitrogen sources. Biomass yield was less at pH 4.8 and 7.1 than at the intermediate pH levels. At pH 7.1, this resulted from the inability of the culture to utilize nitrate. At pH 4.8, the cause of the low yield was not apparent but was probably related to a less efficient utilization of sucrose.The ability of the cells to utilize ammonium increased with increasing pH, with a possible maximum at pH 6.4. The ability to utilize nitrate decreased with increasing pH and was essentially zero at pH 7.1. At pH 4.8, ammonium was released into the medium during part of the growth cycle, indicating that the rate of nitrate reduction exceeded the rate of incorporation of the ammonium thus formed.Utilization of nitrate was not inhibited by the presence of ammonium.