Previous experiments have revealed a much greater efficiency of ammonium utilization by bean plants (Phaseolus vulgaris L.) when the acidity of the ambient medium was maintained at near-neutral conditions with carbonates or hydroxides. The present investigation, in which (15)N-labeled ammonium was used, permitted an assessment of the origin of nitrogen in tissue nitrogen pools with and without acidity control (CaCO(3) treated and untreated, respectively) in the root environment. Control of acidity resulted in greater ammonium uptake and greater incorporation into the amino fraction, amide, and ethanol-insoluble nitrogen by the root tissue. These differences were clearly evident by the fifth day after ammonium nitrogen had been applied.Shoots of the untreated plants rapidly accumulated free ammonium and amino nitrogen. A substantial portion of both fractions came from pre-existing nitrogen in the plants, indicating significant protein degradation. No evidence was found for such degradation in the roots of the untreated plants or in either roots or shoots of CaCO(3) treated plants. The data indicate that control of ambient acidity in the root environment during ammonium absorption enhanced the conversion of entering ammonium to organic nitrogen compounds in the root tissue thereby restricting movement of free ammonium to shoots. Consequently, the detrimental effects of high ammonium concentrations in the leaves were largely prevented.