We evaluated the use of a new lactate oxidase-based reagent for the determination of serum and plasma lactic acid levels with the Hitachi 911 (Roche Diagnostics, Indianapolis, IN) and the Beckman CX7 (Beckman Instruments, Brea, CA). Evaluation studies demonstrated on-board stability of at least 3 months and a calibration stability of more than 5 months. Within- and between-day imprecision of this reagent was less than 2% for both applications. The reagent is free of the deleterious effects of triglyceride up to levels of 1,400 mg/dL (15.8 mmol/L), bilirubin to concentrations of 24.6 mg/dL (420 mumol/L), and hemoglobin, from lysed erythrocytes, to levels of more than 0.3 g/dL (3.0 g/L). When used on the Hitachi 911 for the determination of plasma lactate concentrations, the reagent correlates with the Dade aca III (Dade International, Deerfield, IL). When applied to the Beckman CX7 for the determination of serum lactate levels, the method correlates with the Beckman method.