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Progesterone is critical for the establishment and the maintenance of pregnancy, both by its endocrine and immunological effects. The genomic actions of progesterone are mediated by the intracellular progesterone receptors; A and B. A protein called P‐induced blocking factor (PIBF), by inducing a T_H2 dominant cytokine production, mediates the immunological effects of progesterone. Progesterone plays a role in uterine homing of NK cells and up‐regulates HLA‐G gene expression, the ligand for various NK inhibitory receptors. At high concentrations progesterone is a potent inducer of Th2‐type cytokines as well as of LIF and M‐CSF production by T cells. Though a key role for progesterone in creating local immunosuppression has been conserved during the evolution of an epitheliochorial placenta, there has been some divergence in the pattern of endocrine‐immunological cross talk in Bovidae. In sheep …