Alfalfa (Medicago sativa L. cv. Canadian No. 1), tobacco (Nicotiana tabacum L. var. humilis) and wheat (Triticum monococcum L.) cells were grown in a defined, liquid medium containing either ammonium sulfate, L-glutamine or potassium nitrate as the sole nitrogen source, and the effects of the tricarboxylic-acid (TCA) intermediates, citrate and a-ketoglutarate (5, 10, 15 raM), on the growth (dry-weight increase) of these cells was observed. The three cell suspension cultures exhibited a different growth response to the TCA-cycle intermediate supplied, depending upon the concentration of the additive and the nitrogen source. Citrate (5 raM) greatly enhanced growth of alfalfa and wheat cells in an ammoniumbased medium but was less effective at higher concentrations, and in the case of alfalfa cells markedly inhibited growth. Tobacco cell growth was inhibited by all citrate concentrations tested. In contrast, all concentrations of ~-ketoglutarate used stimulated the growth of all three cell cultures in an ammoniumbased medium. Alfalfa and wheat cells grown in an L-glutamine-based medium were influenced by citrate in a manner similar to that in ammonium-based medium. The growth of tobacco cells was slightly enhanced by 5 mM citrate but inhibited by higher concentrations. ~-Ketoglutarate, at all concentrations tested, was stimulatory to the growth of the cells of all three species in a glutamine-based medium, except for alfalfa cells which were inhibited at 15 raM. Both TCAcycle acids inhibited the growth of alfalfa and tobacco ceils grown on a nitrate-based medium whereas the growth of wheat cells was almost unaffected.