Rearrangement in the c-myb locus of each of four independently derived BALB/c plasmacytoid lymphosarcoma (ABPL's) is due to the insertion of a defective Moloney murine leukemia virus (M-MuLV) into a 1.5-kilobase-pair stretch of cellular DNA at the 5' end of the v-myb-related sequences. This retroviral insertion is associated with abnormal transcription of myb sequences and probably represents a step in the neoplastic transformation of ABPL cells.