Maintenance of the ammonium level in batch cultures of a plant cell line initiated from Ipomoea root tissue resulted in cells with a higher nitrogen content and a greater total accumulation of nitrogen in the cells. Utilization of nitrate was not suppressed. The increased supply of ammonium did not, by itself, increase the yield of dry cells, but it did permit a twofold or greater increase in cell yield when sucrose was added subsequently.Supplementary feeding of ammonium and sucrose to a batch culture that had exhausted the initial supply of ammonium rapidly returned the cells to a state of high nitrogen content, favored further utilization of nitrate, and greatly increased cell yield.These results are discussed in terms of growth phases. The presence of ammonium in the medium appeared to induce and maintain a growth phase characterized by a high nitrogen level in the cells. After the ammonium supply was depleted a second phase occurred, characterized by a rapid increase in dry weight without proportional accumulation of nitrogen. This second phase was not caused by a limiting rate of nitrate utilization, nor was it directly related to observed pH changes. We conclude that the two phases relate to the presence and absence of ammonium in the medium.