Plasmacytoma M603 contains one normal, nontranslocated c-myc gene and one translocated c-myc gene in which c-myc exon 1 is juxtaposed with the immunoglobulin heavy chain enhancer and c-myc exons 2 and 3 are juxtaposed with C alpha. We find that steady-state c-myc RNA levels are 2-4 fold elevated in M603 relative to normal liver or spleen and that these transcripts originate predominantly if not exclusively from the translocated c-myc gene. Although both promoters on the nontranslocated c-myc gene are repressed, the proximal promoter, P1, is active on the translocated 5' c-myc region which is juxtaposed with the immunoglobulin heavy chain enhancer. The 3' portion of the translocated c-myc gene is transcribed from fifteen cryptic start sites and spliced at aberrant donor and acceptor splice sites, thereby generating a mixture of transcripts with different, abnormal 5' untranslated regions. Although the reason that translocation activates the cryptic c-myc starts in M603 is not completely understood, we show that truncation of the c-myc gene is not sufficient to activate cryptic transcription sites.