Objective To investigate the expression of L-selectin mRNA and protein in pregnant mice during the periimplantation period and the effect of L-selectin in the embryo implantation. Methods 1.Mice ovigerms were divided into 4 groups: 1-cell stage, 4-cell stage, morula stage and blastocyst stage. The expression of L-selectin mRNA and protein in mice ovigerms was detected by reaL-time fluorescence quantitative PCR(FQ-PCR) and immunofluorescence histochemistry. The location of L-selectin was analyzed by immunofluorescence histochemistry; 2. Mice were divided into 4 groups: experiment group 1(1∶50), experiment group 2(1∶100), experiment group 3(1∶200) and control group (without any intervention to mice cornu uteris). On the morning of day 4 of pregnancy female mice were injected with 5μl of the L-selectin monoclonal antibody into one horn(experiment side) of uteri and with 5μl normal saline into another horn(control side) in the game mouse. The number of embryos implanted was counted and recorded on day 8 of pregnancy to study the effects of the L-selectin on embryo implantation in vivo. The data were analyzed by analysis of variance. Results 1. With the development and maturation of the ovigerm, the expression of L-selectin mRNA and protein was increased gradually(P<0.05), and the level of L-selectin mRNA on blastocyst stage was higher than those of other groups(P<0.05). There was a little expression of L-selectin protein in 1-cell stage and 4-cell stage. The expression of L-selectin protein which was mainly distributed around the morula increased significantly. The level of L-selectin protein which was mainly distributed on the cell membrane of trophoblast was the highest during the periimplantation period(P<0.05); 2. The number of embryos implanted in the horns treated with L-selectin monoclonal antibody was smaller than those in corresponding controls(P<0.05).Conclusion L-selectin of the ovigerm might particip