The enzyme immunoassay of valproic acid using EMIT reagents was developed for an Abbott ABA-200 analyzer. The number of tests per package that could be obtained using this method was at least three times that suggested by the reagent manufacturer. The reproducibility of the method was assessed by within-run and day-to-day reproducibility studies and the accuracy of the method was assessed by comparison of results obtained to those obtained using a gas-chromatographic method and by performance on an inter-laboratory therapeutic drug monitoring quality control programme.