The long terminal repeats (LTRs) of retroviruses contain sequences necessary for the initiation and termination of retroviral transcription. These sequences include promoter elements, transcriptional termination signals and transcriptional enhancer elements. The enhancer elements of Moloney murine leukaemia virus (M-MuLV) are localized in a tandemly repeated region (approximately 75 base pairs (bp) long), which lies 5' to the CAT and TATA promoter elements in the U3 region of the LTR (see Fig. 1). We have shown that the tandem repeats are required both for LTR promoter activity, as measured by transient expression assays, and for biological activity, as measured by production of infectious virus. Furthermore, they can be replaced by transcriptional enhancers from the F101 host-range mutant of polyoma virus without loss of function. We report here that the addition of the polyoma (PyF101) enhancers to the M-MuLV LTRs (either with or without the M-MuLV tandem repeats) results in complete loss of viral leukaemogenicity, even though the virus can replicate to high titres in tissue culture fibroblasts and can establish infection in animals.