MODEL RELEASED. Microbiologist performing an ELISA (Enzyme-linked Immunoabsorbent Assay) test for antibodies to rubella virus in samples of human blood serum. The ELISA test is performed in a small well on a test plate treated with viral antigens. A patient's blood serum is placed in the well and any antibodies in the serum bind to the antigens. After washing, an enzyme-labelled compound is added which binds to the antibodies. The plate is washed again and a colourless sub- strate, chromogen, added (shown here). The enzyme acts on the chromogen to produce a colour change if antibodies are present. The photo was taken at Newcastle Regional Public Health Laboratory, UK.