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Valproic acid is a very commonly used anti-epileptic drug which requires regular monitoring. The authors consider the assay of this drug by means of 2 techniques: Syva's homogeneous phase immuno-enzymatic technique adapted by Du Pont de Nemours for his automatic clinical analyser and high performance liquid chromatography (HPLC). The precision of the immuno-enzymatic technique is insufficient for concentrations of valproic acid of between 70 and 35 mumol/l (coefficient of variation: 27.0 and 22.0 per cent respectively). The precision of this method for concentrations of valproic acid of 45 and 205 mumol/l is good (coefficient of variation: 8.5 and 8.2 per cent respectively). For concentrations of valproic acid of between 347 and 867 mumol/l, the precision of the immunoenzymological is very good and comparable to that of high performance liquid chromatography (coefficient of variation: 5.9 and 3.8 per cent). For all of the above concentrations of valproic acid, the coefficients of variation obtained with high performance liquid chromatography were less than 5 per cent. Comparison by means of the t test for paired series demonstrated a significant difference between the results of the 2 techniques, with a risk of error of p less than 0.01. The coefficient of correlation between the 2 techniques was 0.96. However, Eksborg's representation shows a difference between the 2 assay methods for concentrations of valproic acid of less than 200 mumol/l, with higher values with HPLC.