Enzyme-linked immunosorbent assays (ELISA) utilize antibodies raised against viruses. There are a number of protocols, but the principle is shown in the figure.

First, the virus is bound to the surface of the well of an ELISA plate, then the antibody raised against the virus is added. The antibody is added in surplus so all virus particles will have an antibody bound to it. The surplus of antibody is washed away. The amount of antibody left is then equal to the number of virus particles. The antibody has an enzyme linked to it. A colourless substrate is added to the well. The enzyme converts this substrate into a coloured compound, which can then be measured spectrophotometrically. The more colour, the more virus.