The expression of IK cytokine was investigated in the mouse endometrium during early pregnancy (D1-D7 of pregnancy) and pseudopregnancy using real-time PCR, western blotting and immunohistochemical analysis, and the effects of IK cytokine on embryo implantation were observed by injection with antisense IK cytokine oligodeoxynucleotides in the uterine horn. Our data showed that the expression of IK cytokine mRNA increased gradually from D1 to D4 of pregnancy and reached a peak level at D4 of pregnancy (P<0.05). Western blotting and immunohistochemical analysis revealed that the expression of IK cytokine protein increased gradually from D1 to D5 of pregnancy and reached a peak level at D5 of pregnancy (P<0.05). The expression of IK cytokine in the pseudopregnant uterus was significantly lower compared to that in the normal pregnant uterus and the level of the protein never showed a high peak during the whole pseudopregnancy. The expression of IK cytokine at the implantation site was much stronger than that in the peri-implantation site on Day 5 of pregnancy. After 24 and 48 h of injection with antisense IK cytokine oligodexynucleotides in the uterine horn on D3 of pregnancy (i.e. implantation window), the expression of IK cytokine in the uterus was remarkably inhibited, while the expression of major histocompatibility complex II (MHC II) increased and the number of implanted embryos significantly decreased in the site of uterine horns receiving antisense IK cytokine (P<0.05). These results suggested that IK cytokine may play a crucial role in implantation.