Haptoglobin is an acute phase protein that can be divided into the phenotypes Hp 1 1, Hp 2 1 and Hp 2 2 by different alpha-unities. The main function of haptoglobin is binding of free hemoglobin, however, in the meantime it appears, that in addition it has immunmodulatory properties and modulates lymphocyte and monocyte immune answer (Sadrzadeh and Bozorgmehr 2004). Haptoglobin also influences decidual lymphocytes and their cytokine expression (von Rango 2005). Thereby it is significant for the development of pregnancy which involves important immunological processes (Nauta 2006). The aim of this work was, to analyze haptoglobin protein and mRNA expression by means of PCR, Western blot and immunohistochemistry within the female cycle and the early pregnancy in humans. Moreover, a possible connection between fecundity and the haptoglobin phenotype was examined. Finally, the haptoglobin production was compared in different tissues to rank the expression in endometrium and decidua quantitatively. Haptoglobin mRNA and protein expression could be shown during the whole menstrual cycle. It was predominantly expressed within the stroma. Haptoglobin mRNA as well as the protein expression decreased with the beginning luteal phase and reincreased at the time of a possible implantation, suggesting a specific regulation and function of the haptoglobin in this period. Haptoglobin was also found to be expressed continuously in the early pregnancy up to the 16th pregnancy week. The expression was high at the time of the strongest extravillous trophoblast invasion in the first trimester. With the beginning of the second trimester expression decreased, then the mRNA and protein expression rose again clearly. The immunohistochemistry showed a clear haptoglobin expression in the glandular epithelium and in the stroma. During pregnancy maternal rejection of the embryo must be prevented. At the time of the implantation and in the early pregnancy many leucocytes migrate into the uterine wall. There haptoglobin may exert its function. Haptoglobin restrains antibody activation and proliferation/activation of T-lymphocytes (Oh et al. 1987, Arredouani et al. 2003, Frings et al. 2002). Furthermore, it prevents the migration of macrophages and B-cells in inflammatory tissue (Hanasaki et al. 1995, Nauta et al. 2006) and increases the expression of antiinflammatory cytokines (Arredouani et al. 2001). Thus haptoglobin can suppress a possible immune answer. The exact timing of haptoglobin expression in the female reproductive tract supposes an influence of haptoglobin on the immunological processes during a pregnancy. No significant difference was found concerning the haptoglobin phenotype and genotype between fertile and infertile women. The analysis of haptoglobin mRNA expression in different tissues showed that beside the predominant expression site the liver a high expression could be found also in reproductive organs like ovaries, tubes and testicles. In the endometrium Haptoglobin expression was clearly low, nevertheless, at the beginning of pregnancy expression strongly increased. This also points to the fact that haptoglobin function is important for the success of pregnancy.