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The optimization of the quantitative photometric estimation of the neurotoxin 3‐N‐oxalyl‐L‐2,3‐diaminopropanoic acid (ODAP) after hydrolysis and reaction with o‐phthalaldehyde (OPT) reagent is described. Six modifications of the OPT procedure (as practiced in different laboratories) and the original Rao method were compared in estimating the ODAP content of two cultivars of Lathyrus sativus. Considerable variation in the ODAP content of the same sample was observed even when determined by the same procedure in some cases. The desired reproducibility was observed with the procedure of the Agriculture Canada Research Station, Mordsen, Manitoba, Canada (Campbell method). The use of 0.5 M tetraborate as buffer further significantly improves the reproducibility. The Campbell modification includes a number of controls which reduce variation. The present study recommends the Campbell modification of the OPT method but with tetraborate buffer as standard procedure for the screening of the toxicity of L. sativus cultivars.