Seventeen isolates of retrovirus D/New England have been obtained from three species of macaques at the New England Regional Primate Research Center. Seven of the isolates were obtained from macaques who subsequently died with the macaque immunodeficiency syndrome; other isolates were obtained from macaques with less severe or other forms of illness. Attempts to isolate type D retrovirus from peripheral lymphocytes of 97 apparently healthy macaques have not been successful. Cloned DNA was prepared from Hirt supernatants of cells infected with one of these isolates (D/New England 398). By restriction endonuclease analysis, cloned pD398 DNA represented full-length viral DNA with one long terminal repeat. A detailed restriction endonuclease map of pD398 was derived and compared with a map of the cloned Mason-Pfizer monkey virus genome. Forty-six percent (13 of 28) of restriction endonuclease sites were found to be conserved when these related viruses were compared. Five of the D/New England isolates, including those from three different macaque species, were examined for strain variability by restriction endonuclease typing. Comparison of over 30 restriction endonuclease sites has not distinguished any of these D/New England isolates. It thus appears that a single strain of type D retrovirus is infecting three different species of macaques in the New England colony. Markedly reduced cross-hybridization was observed between cloned pD398 and Mason-Pfizer monkey virus DNAs at high stringency; this reduced cross-hybridization was localized to the pol-env regions of the genome. Only very weak hybridization of D/New England DNA to cloned squirrel monkey type D retrovirus DNA could be detected even at low-stringency conditions. What role type D retrovirus plays in the immunodeficiency syndrome of macaques remains to be determined.