Objectives To explore the relationship between CD80 expression on lymphocytes at the fetomaternal interface and the fertility characteristics in CBA/J×DBA/2 mice as a model of recurrent spontaneous abortion (RSA) and to investigate the effects of lymphocyte immunotherapy (LIT) on the level of CD80 expression. Materials & Methods The characteristics of fertility in CBA/J×DBA/2 mice were observed in a 120 day period and compared with four normal fertile groups. In another 15 pairs of CBA/J×DBA/2 breedings, resorption rate on day 13 of pregnancy were calculated and the proportion of CD80 + cells at the fetomaternal interface were determined by using two color flow cytometric analysis, mainly stained with CD80 FITC and CD45 PE. In order to determine the identity of CD80 + cells, the expression levels of CD3(T cell marker), DX5(NK cell marker), and MHC II(antigen presenting cell marker) were detected in this cell population. Furthermore, the resorption rate and the proportion of CD80 + cells among CBA/J×DBA/2 breedings with and without immunotherapy were also determined and compared with normal fertile controls. Results The characteristics of abortion in CBA/J×DBA/2 mice were recurrent abortion on about day 10 of gestation. The resorption rate in CBA/J×DBA/2 mice was significantly higher than that in BALB/c×DBA/2 mice (30.8%±16.6% vs. 7.7%±6.7%, P<0.01). Accordingly, the proportion of CD80 + cells evaluated at the fetomaternal interface in CBA/J×DBA/2 mice was also significantly higher (11.7% ±5.8% vs. 3.9%±1.8%, P<0.01). Resorption rate of CBA/J×DBA/2 mice underwent of LIT was significantly lower than that without LIT, and this decreased rate was correlated with decreased proportion of CD80 + cells. Conclusion In CBA/J×DBA/2 mice model, the characteristics of abortion seem to be peri implantation embryo resorption. A correlation between early embryonic waste and higher CD80 proportion at the fetomaternal interface suggests that CD80 + cells may be an important determinant in recurrent peri implantation abortion.