The test uses 96-wells microtitration plates sensitized by a preparation of bovine kidney cells containing the PI-3 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-labelled 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 PI-3 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 endenders 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 (PI-3 – wells, even column) is subtracted from that of the positive microwell sensitized by the viral antigen (PI-3+ 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.