An analysis was made of the retroviral integration sites for retroviruses in a murine lymphoid precursor cell line, C1-V13D, derived following in vitro infection with RadLV, an ecotropic murine retrovirus. A genomic library was constructed and λ clones were selected for their capacity to hybridize with the specific RadLV gp70 ecotropic env probe. Analysis of these clones by a combination of approaches, including subcloning, partial restriction mapping and sequencing, has confirmed the existence of multiple recombinant and defective viruses in C1-V13D. To check for the presence of coding sequences in flanking genomic DNA, 32P-labelled cDNA from C1-V13D was used to probe Hindlll- and Pstl-digested virus-positive λ clones by Southern analysis. Regions hybridizing specifically with 32P-labelled C1-V13D cDNA were subcloned and analysed. A notable feature of these cDNA+ regions was the frequent presence of B1, B2 and simple repeats. These repeat elements were found to be present in high frequency in the genomic regions flanking the proviruses, in numbers higher than expected for the genome as a whole. All full-length viruses isolated appeared to represent integration events into regions rich in repeat elements. Some B1 and B2 repeats have been shown to code for functional proteins and to play regulatory roles. Viral integration in the vicinity of these genetic elements could contribute to oncogenesis if the integration event were to disrupt normal gene function.