We have characterized a chromosomal translocation in a cell line (SU-DUL5) established from a patient with lymphoblastic lymphoma in which the c-myc gene on chromosome 8 was juxtaposed to a t(14;18). Cytogenetic analysis of this cell line showed 14q+, 18q-, and 8p+q+ marker chromosomes in the absence of t(14;18). Genomic Southern blot analysis showed juxtaposition of the immunoglobulin heavy chain joining region (JH) with chromosome 18 near the minor breakpoint cluster region (mcr) of the bcl-2 gene. There was also a rearranged c-myc gene detectable with a 5' c-myc probe. Molecular cloning studies showed that the c-myc gene was joined to chromosome 18 DNA. Nucleotide sequence analysis of cloned breakpoint DNA revealed that the crossover between chromosomes 8 and 18 occurred at the 3' end of the bcl-2 gene resulting in replacement of the bcl-2 gene on the 14q+ chromosome with the c-myc gene. As a result of this translocation the SU-DUL5 cell line contains no detectable bcl-2 mRNA or protein but has abundant levels of c-myc mRNA. Our data suggest that bcl-2 inactivation occurred simultaneously with c-myc translocation in a B cell lymphoblastic lymphoma.