Previous studies have demonstrated both an elevated level of cellular myc-homologous RNA in HL-60 cultures and a decrease in this messenger RNA following the chemically induced differentiation of HL-60 cells. A nuclear transcription system isolated from HL-60 cells was used to investigate whether an alteration in the rate of transcription of the c-myc gene was associated with this decrease in myc RNA. Five days after the addition of either 180 mM dimethyl sulfoxide or 60 nM 12-O-tetradecanoylphorbol-13-acetate to HL-60 cultures, transcription of the c-myc gene was markedly reduced when compared with control cultures. This specific decrease was not accompanied by an alteration in either the bulk rate of transcription of the c-myc copy number. This suggests that the decreased cellular myc RNA levels are due to decreased transcription of the myc protooncogene.