An antisense myc plasmid, containing a portion of exon 2 of the human c-myc gene, was cloned into the pDOL retroviral vector which was used to transfect human K562 erythroleukemia cells. All the retroviral vector transformants showed slower growth rates than the parental K562 cells. No detectable difference in the sense myc RNA level was observed, but the reduction of myc protein levels in the antisense myc transformants varied between 21% and 96%. Most antisense myc transformants contain higher percentage of hemoglobin-producing cells, but no proportionate correlation with the myc protein levels was observed. The results suggest that myc protein is necessary but not sufficient for the regulation of differentiation and growth arrest of K562 cells.