A revised automated method was used to determine ascorbic acid in one hundred random patients' serum samples, and a range of concentrations between 0.7--2.5 mg/dl (0.040--0.141 mmol/L) was found. Volunteers taking a 2000 mg dose of ascorbic acid orally were found to have attained serum levels of ascorbic acid less than 2.8 mg/dl (0.159 mmol/L) in a period of 120 minutes. Five and ten mg of ascorbic acid or its oxidation products were added to 100 ml of pooled serum and the following components were estimated on our SMA 12/60: calcium, inorganic phosphorus, glucose (Trinder), urea, uric acid (phosphotungstate), total protein, albumin (BCG), total bilirubin, cholesterol (enzymatic), alkaline phosphatase, LDH (UV), and SGOT (UV), as well as glucose on a separate Technicon AA-II analyzer (ferricyanide method). With the 10 mg amounts the following analyses were found to be influenced significantly: glucose (Trinder), glucose (ferricyanide), uric acid, cholesterol and total bilirubin; the last of these by the oxidation products only. However, deviations produced for all but one of these tests were less than their allowable limits of error and were therefore considered to be clinically insignificant. The exception was the enzymatic glucose method of Trinder which was sensitive to a plasma ascorbic acid level as low as 2.0 mg/dl (0.114 mmol/L).