The aim of the present study was to investigate the expression profiles of Interferon-tau (IFN-T) stimulated genes (ISGs) at the mid-luteal stage of the cycle (on day 13) in the uterine endometrium and extra-uterine tissues, such as the corpus luteum (CL), and in the peripheral blood leukocytes (PBLs) of pregnant and non -pregnant ewes, The objective was to evaluate the possibility of using the regulation of ISG expression in PBLs as a possible early pregnancy indicator in ruminants. For this purpose, multiparous ewes were synchronized and either allowed to mate (pregnancy group) or detected in estrus (cyclic group; day 0). The ewes were slaughtered on day 13, and the PBLs, endometrium and luteal tissues were collected. Total RNA was isolated from eight cyclic and eight pregnant ewes, and qPCR was employed to detect the steady state levels of Interferon -stimulated gene 15 (ISG15), Myxovirus (influenza virus) resistance 1 (Mxl) and Receptor transporter protein 4 (RTP4) mRNAs. The expressions of ISG15, Mxl and RTP4 were detected in the endometrium, CL and PBLs on day 13 of the estrous cycle and pregnancy. The expressions of these ISGs were upregulated only in the endometrium of pregnant ewes compared to non-pregnant ewes, but this stimulation was not observed in the CL and PBLs. The results suggest that the embryo stimulates ISGs only in the endometrium, and the effects are not evident in the extra-uterine tissues on day 13 of pregnancy. This study suggests that the measurement of the ISG expression in the PBLs is not a reliable detection method of early pregnancy in ewes, which are in the preluteolytic stage of early pregnancy. (C) 2016 Elsevier B.V. All rights reserved.