Early pregnancy factor (EPF) is a pregnancy protein, which is secreted into the maternal serum 12-16 hours after fertilization. It is thought to be an immunosuppressive molecule. EPF is detected in pregnant woman's serum by the rosette inhibition assay (RIA). In this study, EPF was purified from the pregnant woman's sera by using ion exchange chromatography and analyzed by sodium dodecyl sulphate polyacrylamide gel electrophoresis (SDS-PAGE). The proteins which showed a positive result with the RIA, were found to be 35 kDa and 17 kDa molecular weights. The biological activities of these proteins were stable upon heat treatment at 56 degrees C for 30 min. Proteins isolated and purified in this study might be of great significance to the field of human reproduction with particular reference to pregnancy and recurrent abortion.