The Abelson and Moloney murine leukemia virus complex (A-MuLV/M-MuLV) induces rapidly growing thymic lymphomas following direct injection into the thymus of newborn BALB/c and C57BL/6 mice. Southern blot analysis with a v-abl specific probe not only demonstrated that primary tumors are clonal, but also that the pattern of A-MuLV provirus integration is quite stable in primary tumor cells, as well as in their derived cell lines and clones. Most of the cell samples were able to rearrange the immunoglobulin heavy chain genes in culture, whereas in two cases the T cell receptor gamma chain genes also underwent rearrangement. Since the recombination mechanism is operative only in very immature lymphoid cells, these data provide indirect evidence for the lack of differentiation of A-MuLV cell targets in the thymus.