Diagnostic technique in which serum samples containing antibodies are placed in microtitre wells that are precoated with specific microbial antigens. If the patient has antibodies against that antigen, specific binding occurs, and unbound material is then washed out of the wells. Next, an anti-human antibody is added, and this binds to any of the patient's antibodies that remain in the well. Again, unbound material is washed out. The second antibody is attached to an enzyme that catalyses a colour change when its substrate is added. Thus colour changes in the microtitre wells indicate that the patient's serum contained specific antibody and the intensity of the colour reaction can be used for relative quantification.