Most spontaneous abortions occur before 12 weeks'gestation, and most are due to chromosomal errors in the conceptus. Relatively few truly spontaneous abortions take place between 12 and 20 weeks’ gestation. Thereafter, between 20 and 30 weeks another type of premature spontaneous termination due to ascending infection becomes prevalent. The number of cells expressing the various Iymphocytic markers changes throughout pregnancy. In the present study, we investigated the immunohistochemical expres­sion of mononuclear infiltrations in paraffin-embedded placentas, from fetuses after spontaneous abortion (8th, 10th, and 12th week of gestational age), and those after therapeutic abortion at the same time, using a panel of monoclonal antibodies for the identification of leukocytes (CD45/LCA), B-lymphocytes (CD20/L-26), T-lymphocytes (CD45RO/UCHLI) and CD5 cells. Immunologic factors in human reproductive failure are plausible mechanisms of infertility and spontaneous abortion Approximately 25% of cases of premature ovarian failure appear to result from an autoimmune etiology. Unfortunately, current therapeutic options for these women are limited to exogenous hormone or gamete substitution. Local inflammation at the sites of endometrio­sis implants are postulated to mediate the pain and reduce fecundability associated with this clinical syndrome. The recruitment of immune cells, particularly monocytes and T cells, neovascularization around foci of invading peritoneal lesions, and the possible development of antiendometrial autoantibodies support an immunologic basis of this disorder. To date, treatment of pain and infertility associated with endometriosis is primarily surgical, although immune-based adjuvants are theoretical possibilities for the future. Finally, although hypotheses supporting immunologic mechanisms of recurrent pregnancy loss have been popular over the past decade, most clinical investigations in this area do not provide compelling evidence for this position. Reputable specialists in repro­ductive medicine use experimental immunotherapies judiciously in selected cases of repetitive abortion. For example, the use of anticoagulation therapy can be beneficial in cases with documented antiphospholipid antibodies. At present, however, efficacious immunotherapy protocols for general application have not been established. Despite these caveats, continued strides in our under­standing of human reproductive immunology, should yield considerable future progress in this field. We conclude that, 1) maternal cells, probably CD45RO/UCHL 1 positive cells, cross the maternofetal barrier and participate in spontaneous (involuntary) abortions, 2) a small proportion of maternal cells (approximately 30%), probably CDS positive cells, also cross the maternal fetal barrier and cause growth delay and recurrent reproductive failure. The results were statistically significant (p < 0.0001, Student’s t-test).