The concentration of eCG in pregnant mare serum at different days of gestation has been determined by bioassay (A-Z test). The studies revealed that 0.2 ml volume of pregnant mare serum (PMS) is good enough for inoculation in immature mice to produce sexual maturity. It has also been depicted that the concentration of equine chorionic gonadotrophin (eCG) in serum of pregnant mares is nil till 30th day of gestation. The eCG level rises in serum by 45th day of gestation, reaches a peak of 62.5 IU/ml serum by 90th day and declines thereafter to a concentration of 15.0 IU/ml by 135th day of gestation. The eCG activity is maximum during 60–90 days of gestation in PMS.
