Abstract: To study the immunology mechanism of circulating antibody detected in sheep infected ISRV. Preparing the enJSRV-env probes, and we analysed the expression of enJSRV mRNA in immune organs (including thymus, spleen, gut lymph node) and lung of gestation 70 d fetal lambs, gestation 130 d fetal lambs and 7 d newborn lambs by In Situ Hybridization. The expression level of each target mRNA was analyzed in total RNA obtained from the above tissues by Real-Time PCR. The results revealed that enJSRV mRNA expressed in all collected tissues of different periods, while there were no positive signals in negative control. The results of Real-Time PCR showed that the level of enJSRV mRNA was much higher in immune organs than that in lung, especially in thymus and spleen of gestation 130 d fetal lambs and 7 d newborn lambs. This study will unravel the pathogenesis that sheep are tolerized towards infection by the exJSRV.