Progesterone is a one-step immunoassay, based on the principle of the competitive method. Progesterone present in the sample and the labeled enzyme-progesterone in the conjugate compete for binding to the capture antibody on the anti-progesterone coated micro plate. The en- zyme activity in the antibody-bound fraction is inversely proportional to the native progesterone concentration.The unbound components are removed by washing. After addition of the solution containing TMB and hydrogen peroxide, the wells with bound conjugate develop a blue color which is converted to yellow after the reaction has been stopped with sulphuric acid. The color intensity is inversely proportional to the concentration of the progesterone in the specimen and can be read at 450 nm.