Twenty cases of Japanese non-Hodgkin's lymphoma with B cell markers were studied with respect to their immunoglobulin heavy (IgH) chain gene loci on chromosome 14 and BCL2 loci on chromosome 18. All of the 20 cases showed IgH gene rearrangement on at least one of the alleles. Molecular analysis using two chromosome 18-specific DNA probes (pFL1 and pFL2) showed rearrangement of the BCL2 region in three of 20 cases. As these three samples showed the rearranged allele of the IgH gene comigrating with that of the BCL2 gene, polymerase chain reaction (PCR) was carried out to amplify the juncture between IgJH on chromosome 14 and BCL2 on chromosome 18. In all of these three cases, gene amplification between the two loci was found to occur, indicating that molecular mechanisms involved in this type of chromosomal translocation in Japan might be similar to cases in the United States (U.S.) in a limited number of cases. Furthermore, this amplification technique revealed minimal tumor cells circulating in peripheral blood, even though lymphoma cells could not be identified morphologically. Thus, the PCR technique is useful for both the detection of minimal tumor cells as well as for the determination of the molecular nature of chromosomal translocation in some of Japanese cases.