The test uses 96-wels microtitration plates sensitized by a preparation of bovine kidney cells containing the BRSV virus. The plate’s odd columns (1, 3, 5, 7, 9 and 11) contain the virus, whereas the even columns (2, 4, 6, 8, 10 and 12) are sensitized by the saturation protein. We thus have a genuine negative control to differentiate the specific anti-viral antibody from the antibodies directed against others antigenic determinants. Using such a control reduces the number of false positives considerably. The test blood sera are diluted in the buffer for dilution. The plate is incubated and washed, then the conjugate-a peroxidase-labeled anti-bovine IgG1 monoclonal antibody-is added to the wells. The plate is then incubated a second time at room temperature and washed again and the chromogen tetramethylbenzidine (TMB) are added. If specific BRSV immunoglobulins are present in the test sera the conjugate remains bound to the microwells that contains the viral antigen and the enzyme catalysis the transformation of the colorless chromogen into a pigmented compound. The intensity of the resulting colour is proportionate to the titre of specific antibody in the sample. Acidification of the wells stops the reaction. It end enders a shift of the colour from blue to yellow, that allows reading of the optical density at 450 nm. The signal read off the negative control microwell (BRSV- wells, even column) is subtracted from that of the positive microwell sensitized by the viral antigen (BRSV+ wells, odd column). Comparison of the signals obtained for the unknown samples with those obtained for a reference serum supplied with the kit make it possible to rank the serum’s positivity on a five-point scale.