We evaluated the determination of ammonia used in the Du Pont Automatic Clinical Analyzer (aca). Correlation studies were done with a manual method, based on the same enzymatic principles as the aca and with a method involving a cation-exchange resin, followed by the Berthelot color reaction. In either case, the results agreed well with the results found with the aca. For the aca the mean analytical recovery of ammonia in plasma samples was 104%. Within-run and day-to-day precision had coefficients of variation of 6% for normal values and 1 and 2% for above-normal values. The linearity, tested with primary standards and plasma samples, was good up to 1000 mumol of NH3 per liter.