A new family of human endogenous retroviral sequences was recently discovered by way of its relationship to the simian sarcoma-associated virus (SSAV). One molecular clone, termed S71, contains sequences related to the genes coding for the group-specific antigens (gag) and polymerase (pol) proteins of SSAV. At the 3' end of this human retroviral element we have now found a 535-bp region which shows features characteristics of a retroviral long terminal repeat, including potential signal sequences essential for transcriptional control. By means of Southern blotting and in situ hybridization, the sequence was mapped to chromosome 18 band q21.