A full-length human endogenous provirus termed ERV3 was isolated from a human fetal recombinant DNA library by low stringency hybridization with two probes: baboon endogenous virus LTR; and a pol-env subclone from the endogenous chimpanzee provirus, CH2. DNA sequencing within the clone and comparisons with other retroviruses revealed that ERV3 contains gag and pol gene sequences that are significantly related to those of mammalian type C retroviruses and previously described human endogenous proviruses. The ERV3 genome was determined to reside at a single locus on human chromosome 7 using a panel of rodent X human somatic cell hybrids.