Using the Southern procedure, we have studied the presence of ecotropicspecific murine leukemia viral sequences in genomic DNA isolated from primary X-ray-induced thymomas, from lymphoid cell lines established from them, or from secondary tumors passaged in vivo. We found that primary radiation-induced thymomas and infiltrated spleens do not harbor newly acquired ecotropic provirus. However, additional ecotropic proviruses (which appear recombinant in the gagpol region) could be detected in most of the tumorigenic cell lines established in vitro from them and in tumors arising from subcutaneous transplantation of the primary thymomas. These results suggest that primary radiation-induced thymomas may not be clonal. They also indicate a strong correlation between the presence of ecotropic recombinant proviruses in the genome and the growth ability, both in vitro and in vivo, of specific cells within these thymomas, suggesting a possible mitogenic function for murine leukemia virus.