G-banding analysis of seven Ig-secreting spontaneous rat immunocytomas showed a consistent translocation of the distal part of the q-arm of chromosome 7 to the telomerk end of chromosome 6. The breakpoints were assigned to q3.3 on chromosome 7 and q3.2 on chromosome 6. Previously, we found a similar translocation pattern in mouse plasmacytomas induced by different agents. The distal part of die q-arm of chromosome 15 was translocated to the telomerk end of chromosome 12, known to carry Igh, the immunoglobulin heavy chain cluster. The banding homologies between the chromosomes involved in the translocations in the two species suggest that a similar mechanism is responsible for plasmacytomagenesis in both. We also predict that the rat Ig heavy chain gene cluster will be located to the terminal segment of chromosome 6.