The authers employed combinations of sequential (NH4)2 SO4 precipitation, DE-52 ion exchange chromatography,Sephadex G-200 gel filtration to isolate SP1 from retroplacental blood collected during labour. Anti-SP1 affinity chromatography has proven helpful in purification. The final product was shown about 207 fold of purification. Purified SP1 reacts with anti-SP1 serum from Behringwerke AG, migrates as p globulin in immunoelectrophoresis. SDS polyacrylamide gradient gel electroph-oresis indicates a major band with molecular weight around 90000. The antiserum obtained by immunization of rabbits with purified SP1 after appropriate absorption shows a single peak in cross immunoelectrophoresis, fails to react with serum from normal nonpregnant females and males, yields single line showing identity with SP1 antiserum from Behringwerke AG in immunodiffusion with pregnant plasma. This antiserum has been successfully used in establishing ELISA for SP1