Several studies have focused their attention on the possible interferences of the endocrine with the immunity system; these interrelations have been summoned to explain some aspects of the implantation. It has already been demonstrated that progesterone could play an immunosuppressive role, allowing the implantation of allotransplantation. However, the individual plasmatic levels of the substance needed to produce that effect still unknown. With the aim of determining the certain immunosuppressive progesterone levels in women with normal ovulatory function, and in order to determine if the other principal steroids might also have immunosuppressive effects, in 47 women affected by sine causa infertility, treated with 100 mg. die of Clomiphene Citrate (from day 3 to day 7) we evaluated the plasma levels of progesterone, 17-OH-Progesterone and 17 beta-Estradiol. The assays were made on the 7th, 11th and 14th post-ovulation days both in women who conceived (immunosuppressive effect present) and in women who did not achieve pregnancy (immunosuppressive effect absent). The results achieved showed a significant difference only in the progesterone values, while those of the other steroids were not significantly different, indicating thus that progesterone is the main element responsible for the immunosuppressive phenomenon and that the seriated evaluation in the luteal phase of this steroid could be used as a marker of achieved implantation.