We applied a simple polymerase chain reaction (PCR) based method for detecting immunoglobulin heavy-chain (IgH) gene rearrangement, using its CDR-III region to assess B-cell clonality in a series of 100 acute lymphoblastic leukemias (ALL) (84 B-cell lineage, 4 null-ALL and 12 T-ALL). The amplified CDR-III regions can be generated in all the B-lineage ALL and separated by size by polyacrylamide gel electrophoresis (PAGE), thereby providing a specific diagnostic marker for each B cell clone. Size heterogeneity resulting from independent IgH rearrangement events and the high resolution power after electrophoresis and silver staining of the PAGE gels can be used to generate a "fingerprint" of the PCR fragments representing either the spectrum of B-cell clonality in complex populations of B lymphocytes or the partially genomic configuration of the VH-N-DH-N-JH region. At diagnosis, we found the presence of one clonal IgH heavy-chain gene rearrangement in 80 B-cell lineage and null ALL and a biclonal rearrangement in 8 cases. The CDR-III bands were of sizes ranging from 80 to 130 bp. The PCR analysis of the IgH gene enabled us to obtain a CDR-III leukemia specific product in all cases, thereby providing a specific and diagnostic marker for each B-cell clone.