Summary 1. A specific assay for the measurement of the plasma concentration of oxalic acid has been developed. The assay procedure consists of isolation of acidic components from the plasma by adsorption onto ion exchange resin, elution with acid, and conversion of the acids to methyl esters. The esters are measured by gas chromatography. Oxalic acid labeled with 14C is used as an internal standard. 2. Recovery of added oxalic acid was 69 ± 4%. The standard deviation for the assay of standard solutions was 6.7%. None of the low molecular weight organic acids known to be present in plasma interfered with the measurement of oxalic acid. 3. Twenty fasted human subjects were found to have plasma oxalic acid levels of 14.5 ± 8.9 μg/ml. This value is ten times those reported by other workers. Study of plasma protein binding of oxalic acid showed extensive binding at pH 6 and below. This suggests that previously reported assays involving acid deproteinization probably result in extensive losses of oxalate bound to protein.