The heme enzyme myeloperoxidase (MPO), with a spectral A430/A280 ratio > 0.78, was purified from isolated bovine neutrophils. Using highly specific anti-MPO monoclonal and anti-MPO polyclonal antibodies raised against MPO, a specific and sensitive double-antibody enzyme-linked immunosorbant assay (ELISA) was developed to measure bovine MPO in serum and neutrophil extracts. The ELISA shows good precision and accuracy, with intra- and interassay coefficients of variation of < 10% for MPO concentrations ranging from 0.5 to 50 ng/ml. The accuracy of the ELISA for measuring MPO in bovine serum was further confirmed by the similarity between the standard curve and curves obtained with successive dilutions of MPO-rich serum samples. The mean analytical recovery of MPO from serum was approximately 90%. Long delays between blood sampling and serum preparation were found to affect the level of MPO in the serum. Mean MPO values in the serum of healthy adult cows were 6.5 ng/ml, with a range of 3.5-15.3 ng/ml. In dairy cows with acute mastitis, mean serum MPO values were approximately 30 ng/ml, with a range of 6.0-59.6 ng/ml, and the elevation was markedly higher than the normal values (P = 0.0001). In isolated neutrophils from healthy cattle, MPO concentrations were found to be 7 x 10(-4) ng, with a range of 6.5-8.3 x 10(-4) ng/neutrophil. The ELISA was used to study the distribution of MPO in the bovine neutrophil granules; it was found to be localized to one distinct compartment.