Two sets of mutants of the Abelson murine leukemia virus, generated by linker insertion mutagenesis of a cloned proviral DNA, were tested for their ability to transform bone marrow cultures in vitro. All the viruses retained an intact tyrosine kinase domain and were competent for transformation of NIH3T3 fibroblasts in culture. One series contained 12-bp linker insertions in the regions flanking the kinase domain, and the other contained frameshift mutations that truncated the gene product downstream of the kinase domain. The majority of the 12-bp insertion mutants retained full bone marrow-transforming activity; only one insertion in the SH2 domain showed reduced activity. This mutant suggests that some aspect of the SH2 domain may be more important in transformation of lymphocytes than fibroblasts. In contrast to the first set of mutants, the bone marrow-transforming activity of the majority of the truncation mutants was significantly reduced or completely lost. We conclude that there is a broad requirement for an intact C-terminal domain of the v-abl protein for the transformation of pre-B cells, but that no single part of this domain is critical.