The chromosomes of unstimulated and stimulated blood lymphocytes from 5 cases with B-cell prolymphocytic leukemia (PLL) were examined following the use of polyclonal B-cell activators (PBA). Banding techniques revealed a common and specific chromosome abnormality to be present in each of the cases, which was due to a translocation between chromosomes 6 and 12 (t(6;12)(q15;p13]. The fact that this specific chromosome change has not been reported in other lymphoproliferative disorders may indicate that PLL is a distinct clinical entity and different from other lymphoproliferative disorders, whether it occurs de novo or complicates chronic lymphocytic leukemia (CLL).