Summary A radioimmunoassay for α1-fetoprotein in human maternal serum at 15–20 weeks' gestation using polyethylene glycol to separate bound and free antigen is described. A number of variables has been investigated to define optimal conditions for use at this gestation. The method gives results which agree with those from a double-antibody technique. It has good precision in the critical concentration range, and recoveries are satisfactory. The importance of including sufficient serum protein in the analytical system is stressed.
There were systematic differences between three standard preparations of α1-fetoprotein, but three out of four antisera tested gave very similar results. The implications for screening for neural tube defects in early pregnancy are discussed.
