The recent development of the polymerase chain reaction (PCR) has enabled us to determine the hypervariable sequence of immunoglobulin heavy chain known as complementarity determining region (CDR)-III. We amplified the leukemia-specific CDR-III from common acute lymphoblastic leukemia (cALL) cells using the PCR and determined its sequence. To detect minimal residual leukemia (MRL) cells, a second round PCR was performed with clone-specific primers corresponding to 5' and 3' ends of CDR-III to detect MRL cells. A million-fold diluted leukemia cells were clearly detected. Using stepwise diluted materials, the number of residual cells was semiquantitatively estimated. In one patient with cALL, induction chemotherapy resulted in a hematologically complete remission with only a 2-log reduction of the leukemia cells. MRL cells at a level of 10(-6) were also detected within 1 month after bone marrow transplantation (BMT), but leukemia cells were not detectable 6 months after BMT.