Abstract: Method of quantitative analysis of an enzyme linked immuno assay comprising the steps of forming an antigen--antibody complex between an antigen present in a sample to be tested and an enzyme labelled antibody and thereafter adding a substrate composition which can be converted into a differently colored product composition by the catalytic action of the enzyme. Quantification is achieved by measuring the time elapsed from addition of the substrate composition until a sudden color change discernible to the eye occurs. The measured time is then compared against a predetermined standard to determine the amount of antigen present in the sample.