We determined the maternal peripheral lymphocyte response to mitogen during the period of implantation and evaluated the effects of hormones, which are known to be involved in the process of implantation, on the lymphocyte activity in rabbits. As compared with peripheral lymphocyte activity in non-pregnant rabbits, lymphocyte activity was significantly depressed on days 6, 7 and 9 of pregnancy. Although concentrations of serum progesterone were gradually increased during the implantation period, progesterone did not inhibit lymphocyte activity at physiological concentration. Serum PGE2α was significantly increased on day 7 while PGE was slightly increased. PGE2α did not modify lymphocyte activity even with greater concentrations than physiological level. However, lymphocyte activity was significantly inhibited by PGE even with physiological doses. The treatment of indomethacin at doses of 0.1 or 1.0μg/ml tended to enhance lymphocyte response, which was depressed on day 8 of pregnancy, 28% or 23% respectively. Although in non-pregnant rabbit, enhancement of lymphocyte response was also shown after the treatment of indomethacin, this enhancement was much less than that in pregnant rabbits. These results strongly suggest that maternal immune response was depressed during the process of implantation and PGE might be one of factors for immunomodulation during this period.