ABSTRACT: Utilizing rosette inhibition test recommended by Morton et al., (Morton H, Tinneberg HR, Rolfe M, Mettier L. Rosette inhibition test: a multicenter investigation of early pregancy factor in humans, J Reprod Immunol. 1982; 4:251–261) we have examined 44 fetal sera of abortion induced by water bag in the second and third trimester pregnancy, 4 umbilical sera of newborns, and their 48 maternal sera for detecting early pregnancy factor (EPF), which activity was expressed as rosette inhibition titre (RIT). The mean RIT value of EPF in fetal sera was 6.00±0.31 (SD) during 16–17 weeks of gestation, gradually decreased along with gestational weeks. got to 4.25 ±0.25 by term. The mean RITs of EPF activity in maternal sera were 5.86±0.26 and 5.89±0.35 during 5–7 weeks and 18–19 weeks, respectively, decreased afterwards, and then fell into nonpregnant range (RIT: 4.00±0.40) after 31 weeks of gestation. There was a close correlation between RITs of 44 fetal and those of their maternal sera (r = 0.615; P<0.001). The results revealed the presence of EPF in fetal serum during gestation. The significance of EPF distribution and its possible role of immunological regulation on survival of semiallogeneic conceptus have thus been discussed.