An automated method for the determination of plasma 3-hydroxybutyrate (3-HB) is described. The enzymatic principle of Williamson and Mellanby is applied to a discrete computerized analyzer (Isamat, Isa-Biologie, 94230 Cachan, France). Using this method, a single operator can determine each day the 3-HB content of 500-600 bovine plasma samples, without perchloric acid precipitation and without adding hydrazine. Aceto-acetate and acetone don't interfere in 3-HB determination. Addition of oxalate (5 mM) in Tris buffer abolishes the interference of lactate dehydrogenase (LDH). The method is accurate, with a precision of 2.8%, linear over the range 0.05-8 mM, low in cost because it needs small volumes of reagent, and correlates well with results from standard end-point manual method (common variance between the two methods: 94-98%). The method allows to use plasma 3-HB determination for assessing nutritional state of cow herds after a convenient standardization of sampling