We followed the "abbreviated precision protocol" of the National Committee for Clinical Laboratory Standards for the evaluation of precision, accuracy, and carryover in analysis for urea nitrogen with the multilayer film analysis system ("Ektachem"). We analyzed 456 clinical samples with this instrument, by the manual urease/glucose dehydrogenase method, and with the Beckman System I GLU/BUN Analyzer. Precision and accuracy were estimated for 50, 220, 270, and 500 mg/L urea nitrogen concentrations in 100, 30, or 20 microL of serum. Potential interference of 15 compounds was evaluated. Random error (defined as 1.965 X SD) was 7, 10, 12, and 18 mg/L. Systematic error was 3, 4, 5, and 15 mg/L. Total analytical error was 11, 14, 17, and 34 mg/L for analysis of 100 microL of serum at the above-stated urea nitrogen concentrations. The greatest interference (6 mg/L) was caused by ethanol (300 mg/L) and by hemoglobin (500 mg/L) in the urea nitrogen (at 260 mg/L) determination. Urea nitrogen concentration, as determined with the Ektachem was linearly related to the expected concentration, at least up to 1187 mg/L. Carryover was not statistically significant.