The article by B.K. Van Weemen and A.H.W.M. Schuurs describes the development and application of an enzyme-immunoassay (EIA) using human chorionic gonadotrophin (HCG) conjugated to horse radish peroxidase (HRP). The authors highlight the sensitivity and practicality of this method, particularly in endocrinology, where traditional bioassays are often cumbersome and costly. The study involves the preparation of HCG-HRP conjugates, the purification of these conjugates using density gradient centrifugation, and the development of two EIA methods: the solid phase (SP) method and the double antibody solid phase (DASP) method. The results show that the DASP method is more sensitive and provides better duplicate determinations compared to the SP method. The sensitivity of the EIA is comparable to that of hemagglutination inhibition assays, and the method is suitable for clinical applications, especially in urine samples. The study also discusses the importance of separating antibody-bound and free conjugates in EIA and the impact of HCG/HRP ratios on sensitivity.The article by B.K. Van Weemen and A.H.W.M. Schuurs describes the development and application of an enzyme-immunoassay (EIA) using human chorionic gonadotrophin (HCG) conjugated to horse radish peroxidase (HRP). The authors highlight the sensitivity and practicality of this method, particularly in endocrinology, where traditional bioassays are often cumbersome and costly. The study involves the preparation of HCG-HRP conjugates, the purification of these conjugates using density gradient centrifugation, and the development of two EIA methods: the solid phase (SP) method and the double antibody solid phase (DASP) method. The results show that the DASP method is more sensitive and provides better duplicate determinations compared to the SP method. The sensitivity of the EIA is comparable to that of hemagglutination inhibition assays, and the method is suitable for clinical applications, especially in urine samples. The study also discusses the importance of separating antibody-bound and free conjugates in EIA and the impact of HCG/HRP ratios on sensitivity.