The 3'-end host-virus junction fragments from two bovine leukemia virus (BLV)-induced lymphoid tumors (tumors 15-4 and 1351), each containing a single provirus, were used as probes to detect large restriction fragments flanking these proviruses. The DNAs from 28 other independent BLV-induced tumors were checked by Southern analysis of their restriction fragments for possible rearrangement due to the insertion of a BLV provirus in the cellular sequences corresponding to those flanking the proviruses in tumors 15-4 and 1351. In no case did proviral integration occur in cellular sequences corresponding to those implicated in the tumors of origin. According to the statistical analysis performed, if a preferential domain for BLV integration exists, it has a size of 1,304 kilobases when the probability of not observing an integration event in the cellular fragments considered in tumors 15-4 and 1351 is 0.50.