T-cell lymphomas developed in three of 10 immunosuppressed rhesus macaques during early experiments using retroviral vectors to transfer marker genes into CD34+ bone marrow cells for subsequent transplantation in the animals. Direct PCR analyses of RNA obtained from tumour tissues from these macaques revealed the presence of several different recombinant murine leukaemia viruses (MuLV). Most prominent was a recombinant designated Mo(LTR)Ampho(env) in which the amphotropic env of the helper packaging virus was joined to a modified form of long terminal repeat (LTR) of the Moloney MuLV-derived vector that contained an additional copy of the core enhancer. This new LTR afforded enhanced replication upon the Mo(LTR)Ampho(env) MuLV in several different rhesus cell types compared with the prototype amphotropic MuLV4070A. Unexpectedly, at least two types of a mink cell focus-forming (MCF) MuLV element, arising from endogenous retroviral sequences expressed in the murine packaging cell line, were also transmitted and highly expressed in one of the macaques. Furthermore, murine virus-like VL-30 sequences were detected in the rhesus lymphomas, but these were not transcribed into RNA. The unanticipated presence of this array of MuLV-related structures in a primate gene transfer recipient highlighted the generation of recombinant retroviruses when the vector producer line produced replication competent viruses. These recombinants had an enhanced tropism and pathogenicity in the primate gene transfer recipients and frequently caused lymphomas. This primate experiment highlights the potential risk from contamination of a vaccine cell substrate with a replicating retrovirus.