Feline IL-12p40 ELISA kit is an in vitro enzyme-linked immunosorbent assay for the quantitative measurement of feline IL-12p40 in serum (feline IL-12p40 concentration is low in normal serum and plasma samples, and may not be detectable in this assay), plasma and cell culture supernatants. This assay employs an antibody specific for feline IL-12p40 coated on a 96-well plate. Standards and samples are pipetted into the wells and IL-12p40 present in a sample is bound to the wells by the immobilized antibody. The wells are washed and biotinylated anti-feline IL-12p40 antibody is added. After washing away unbound biotinylated antibody, HRP-conjugated streptavidin is pipetted to the wells. The wells are again washed, a TMB substrate solution is added to the wells and color develops in proportion to the amount of IL-12p40 bound. The Stop Solution changes the color from blue to yellow, and the intensity of the color is measured at 450 nm.