Cells from murine plasmacytomas (MPC) consistently exhibit either the 12;15 or the 6;15 translocation. Recently, we found that cells from 2 MPC (ABPC 45 and ABPC 26) did not exhibit any known translocation. However, these MPC contained one chromosome 15 (15q-) that was shorter than its normal homologue and one 12 (12q+) that was longer than its normal 12 counterpart. Chromosome 15q- contained only one thin D band, corresponding in size to either D1 or D3 band. A minor light band D2 was apparently deleted. Chromosome 12q+ contained a light F1 band that was longer than the normal 12F1. In ABPC 45, the c-myc gene was assigned by in situ hybridization to the F1 region of 12q+ as well as to its original site of 15D2. Moreover, the in situ hybridization experiment assigned the S alpha gene to the F1 regions of both chromosomes 12 and 12q+. These findings indicate that a minor light band D2 of chromosome 15 carrying the c-myc gene was translocated into the major light band F1 of 12, resulting in a longer chromosome 12q+ and a correspondingly shorter chromosome 15q-. This type of chromosomal translocation suggests that ABPC 45 developed through a cytogenetic mechanism different from that of MPC with the typical t(12;15) translocation.