The "high lymphoma-prone" baboon stock (Papio hamadryas) of the Sukhumi Primate Center colony is characterized by a high prevalence of antibodies to the STLV-I/HTLV-I type of retrovirus and a high manifestation of human ATL-type (adult T-cell leukemia/lymphoma) malignancies (Yakovleva et al., this symposium). This is in contrast to other primate colonies and wild monkeys, which have low seroprevalence and very few if any ATL-type T-cell malignancies. To characterize the type of T-cell lymphoma retrovirus involved in the Sukhumi disease, a PCR (polymerase chain reaction) DNA analysis of peripheral blood lymphocytes (PBL) and of various tissues of healthy "at-risk", or ill baboons was performed. Proviral STLV/HTLV sequences were detected in all monkeys with symptoms of T-cell malignancy and/or antibodies to STLV-I/HTLV-I. For precise identification and characterization of the Sukhumi T-cell lymphoma virus, parts of the virus genome were mapped and sequenced from PCR derived fragments. A 420 nucleotide fragment of the env (gp 46) gene (analysed from 3 different DNA's) revealed 16.2% nucleotide divergence to the Japanese strain of HTLV-I and 14.8% to the Japanese strain of STLV-I including one deletion of a triplet. On the level of amino acid (a.a) sequence this revealed an exchange of 6 a.a. to STLV-I (4.3%), but only of 4 a.a. to HTLV-I (2.8%). The analysis of 120 nucleotides of the tax sequence (identical in 6 different DNAs) resulted in 5% nucleotide divergence to the HTLV-I (2.4% on the a.a. level) and 10% (7.3% a.a.) to the STLV-1. These results indicate that the Sukhumi T-cell lymphoma virus is a representative of the T-cell leukemia/lymphoma virus family, apparently more closely related to HTLV-I than to STLV-I genome. Furthermore, the infected monkeys from Sukhumi develop at a high rate a T-cell malignancy not observed among other baboons carrying STLV.