Endogenous retroviral sequences have been identified in the genomes of several species including humans. Proteins similar to those of primate endogenous viruses have been found on the surface of various malignant cells in man. To further define this role, we have used a primate retrovirus DNA from the Baboon Endogenous Virus to probe a human genomic library for related sequences. A total of 45 clones homologous to BaEV gag-pol were isolated under low stringency hybridization. Of these, several were found to contain DNA which was expressed as RNA at higher levels in human lymphoid leukemic cells than in normal lymphocytes.