Anti-progesterone monoclonal antibody injected intraperitonially as a single dose(100$mu extrm{g}$) 48hours post coitum(p.c.) almostly blocked pregnancy in ICR mice. The blocking rate of pregnancy in mice treated with antibody were decreased proportionally according to dose of antibody injected ; the rate were 60%, 57% and 17% as the antibody of 10$mu extrm{g}$, 50$mu extrm{g}$ and 100$mu extrm{g}$ were injected respectively. Blood serum progesterone concentration was greatly increased(21 times) after treatment(100$mu extrm{g}$), by virtue of high-affinity binding by antibody in circulation of non-pregnant mice in coompared with that of control group at day 10 p.c.. The concentration was about 1.6 times higher in the pregnant mice than in the non-pregnant mice in antibody treated group. In control group, the progesterone concentration was over 7 times higher in the pregnant mice than in non-pregnant mice at day 5 p.c..