Premature termination of transcription is assumed to be an important mechanism of c‐myc regulation. Induction of terminal differentiation in the promyelocytic leukemia cell line HL60 by dimethyl‐sulfoxide (DMSO) is accompanied by a block of RNA elongation within the first exon of the c‐myc gene. We have studied the 3′‐structure of incompletely elongated transcripts in (i) nuclear RNA of induced and uninduced HL60 cells, (ii) nuclear run‐on RNA, and (iii) RNA of in vitro transcribed c‐myc constructs. Elongation of c‐myc RNA stopped in all three transcriptional systems at similar sites distributed 150–350 bases downstream of the P2 promoter. When HL60 cells were induced to terminal differentiation the short c‐myc exon 1 specific RNAs disappeared in nuclear RNA. This implied that RNA polymerase II (pol II) does not continue to transcribe c‐myc exon 1 in induced HL60 cells as suggested by earlier nuclear run‐on experiments. Therefore, kinetic experiments with small oligonucleotides as probes were performed to determine the start position of pol II on c‐myc exon 1 in nuclear run‐ons. The results demonstrate that all RNA polymerases are localized at the c‐myc P2 promoter in DMSO‐treated HL60 cells. Preparation of nuclei for run‐on experiments induces a release of pol II from the c‐myc P2 promoter leading to the strong nuclear run‐on signal on c‐myc exon 1. Thus, down‐regulation of c‐myc in differentiating HL60 cells occurs by retention of pol II at the transcription start site.