One of the features of the life cycle of retroviruses is insertion of the proviral DNA into host chromosomes. A protein encoded by the 3' end of the pol gene of the virus genome has been shown to possess endonuclease activity (D. P. Grandgenett, A. C. Vora, and R. D. Schiff, Virology 89:119-132, 1978), which is necessary for DNA integration. Sera from the majority of human immunodeficiency virus (HIV)-infected individuals react with endonuclease protein p31 in serological tests (J. S. Allan, J. E. Coligan, T.-H. Lee, F. Barin, P. J. Kanki, S. M'Boup, M. F. McLane, J. E. Groopman, and M. Essex, Blood 69:331-333, 1987; E. F. Lillehoj, F. H. R. Salazar, R. J. Mervis, M. G. Raum, H. W. Chan, N. Ahmad, and S. Venkatesan, J. Virol. 62:3053-3058, 1988; K. S. Steimer, K. W. Higgins, M. A. Powers, J. C. Stephans, A. Gyenes, G. George-Nascimento, P. A. Liciw, P. J. Barr, R. A. Hallewell, and R. Sanchez-Pescador, J. Virol. 58:9-16, 1986). It is not known, however, which part of the protein represents the target(s) for antibody response. To study this, we synthesized peptides and used them in an enzyme-linked immunosorbent assay system to map the reactivity of human immunodeficiency virus type 1 (HIV-1) antibody-positive sera to the different regions of the HIV endonuclease. A uniquely antigenic, HIV-1- and HIV-2-cross-reacting site was identified in the central part of this protein from Phe-663 to Trp-670.