A complex translocation has interrupted the third exon of the c-myc gene in human plasma cell myeloma tumor cells and a derivative cell line (NCI-H929). As a result of this rearrangement, a chimeric mRNA is expressed which commences 5' of the c-myc coding region and includes sequences introduced by the translocation event. All of the detectable c-myc-containing mRNA in the tumor and cell line was derived from this rearranged c-myc allele. This chimeric c-myc mRNA, in which most of the germ line c-myc 3' untranslated region has been replaced, was greater than sevenfold more stable than c-myc transcripts with intact 3' ends. This suggests that the 3' untranslated region may play an important role in c-myc mRNA stability.