We examined the RNA expression of a human endogenous provirus, termed ERV3, in 170 human tissue and cell specimens. The highest expression was found in normal placental chorionic villi as mRNAs of 9, 7.3, and 3.5 kb. The 7.3-kb RNA species was found only in the placenta. ERV3 mRNA was expressed in most other normal and malignant tissues at a level which was 2-10% of that seen in placenta. However, several tissues and tumor cell lines had higher transcriptional levels, equal to 10-60% of the placental level. In contrast, an almost complete abrogation of ERV3 mRNA expression was noted in choriocarcinoma cell lines and in an invasive hydatidiform mole tissue biopsy. This abrogation was not linked to deletions or rearrangements of the ERV3 genome. It appeared to be unassociated with methylation because the ERV3 provirus was similarly methylated in the DNA of placental chorionic villi and choriocarcinoma cells, and ERV3 transcription in choriocarcinoma cells was not induced by 5-azacytidine. These results suggest that the loss of ERV3 mRNA expression is associated with susceptibility to choriocarcinoma.