ERV3 (HERV-R) is a complete human endogenous retrovirus located on the long arm of chromosome 7. It is expressed in several human tissues as LTR env spliced transcripts (9 and 3.5 kb). The highest level of expression is to be found in placenta and virus expression is down-regulated in choriocarcinoma cell lines. By means of in situ hybridization, the expression of ERV3 env was studied in selected human reproductive and embryonic tissues. It is concluded that (a) ERV3 env is expressed in syncytiotrophoblasts not only in the placenta but also in hydatidiform moles and choriocarcinomas (irrespective of origin) (b) ERV3 expression in placenta correlates to cell fusion but probably not to the fertilization process itself (c) ERV3 env is highly expressed in certain cells in spermatogenesis but not in the Sertoli or Leydig cells, and finally (d) ERV3 env is expressed in certain embryonic tissues such as the adrenal gland and nervous tissues.