The concept of using the concentrations of progesterone in milk for pregnancy testing became a commercial reality in 1975. Although this has proved to be a highly efficient method of identifying non-pregnant cows earlier than can normally be achieved by manual examination the results are not available in time for the first return-to-service period. The development of an enzymeimmunoassay technique by MAFF has resulted in the production of simple colour-change tests suitable for use in a practice laboratory or by the farmer in the convenience of his own office. The main advantage of an “on-farm” test is that the results are immediately available to the herdsman.