As the uses for ELISA (enzyme-linked immunosorbent assay) increase, so does the need for a quantitative procedure that does not require a spectrophotometer or other expensive equipment. 'Timed ELISA' employs an 'iodine clock' as the final step such that quantitative measurements may be made using a stopwatch. Catalase, coupled to the primary antibody, reduces the concentration of H2O2 available to generate iodine in the clock reaction. Iodine stains the starch component blue, but catalase prolongs the time taken for the change in colour to be observed. After the time delay occurs the transition to full colour development is extremely rapid (< 1 s) at all analyte concentrations, allowing clear definition of the end point. The performance of Timed ELISA is similar to that obtained using a horseradish peroxidase-conjugated system employing the customary spectrophotometric determination.