Early pregnancy factor(EPF) is one of the pregnancy associated proteins, and it is detected in senun of many pregnant animals shortly after fertilization. The present study was conducted to establish a system of early pregnancy by EPF detection in Hanwoo cows. The EPF activity was measured by the rosette inhibition test on day 15 after artificial insemination or non-insemination. The rate of rosette formation between Hanwoo lymphocytes and sheep red blood cells was significantly(P$lt;0.05) higher compared to that between Hanwoo lymphocytes and goat, or rabbit red blood cells. The rosette formation between lymphocytes suspension and sheep red blood cells with anti-lymphocytes was not significantly(P$lt;0.05) changed until 5∼20 min. of culture, and then decreased dramatically by 30 min. There were significant(P$lt;0.05) differences in the rosette inhibition titer(RIT) between pregnant and non-pregnant Hanwoos on day 15 after artificial insemination. The results indicate that the expression of early pregnancy factor by the rosette inhibition test might be used to diagnosis of early pregnancy in Hanwoo.