Constitutive expression of the c-myc oncogene blocks dimethyl sulfoxide (DMSO)-induced differentiation of mouse erythroleukemia (MEL) cells. During the first 12 h of treatment with DMSO, MEL cells undergo a temporary decrease in the level of c-myc mRNA, followed by a temporary withdrawal from the cell cycle. We found the same shutoff of DNA synthesis during the first 12 to 30 h after DMSO induction in normal MEL cells (which differentiate) and in c-myc-transfected MEL cells (which do not differentiate). We also examined whether deregulated c-myc expression grossly interfered with the regulation of gene expression during MEL cell differentiation. We used run-on transcription assays to monitor the rate of transcription of four oncogenes (c-myc, c-myb, c-fos, and c-K-ras); all except c-K-ras showed a rapid but temporary decrease in transcription after induction in both c-myc-transfected and control cells. Finally, we found the same regulation of cytoplasmic mRNA expression in both types of cells for four oncogenes and three housekeeping genes associated with growth. We conclude that in the MEL cell system, the effects of deregulated c-myc expression do not occur through a disruption of cell cycle control early in induction, nor do they occur through gross deregulation of gene expression.