Treatment of the acute promyelocytic (APL) cell line NB4 with interferon alpha (IFN(alpha)), as well as IFN(beta) and gamma, results in an increased expression of the transcripts coding for retinoic-acid receptor type alpha (RAR(alpha)) and the leukemia-specific retinoic acid receptor PML-RAR. Transcriptional induction of the RAR(alpha) and PML-RAR mRNAs is rapid and it is parallelled by an increase in the corresponding proteins. Up-regulation of RAR(alpha) and PML-RAR gene expression by IFN(alpha) is accompanied by a strong potentiation in the induction of 2 retinoid-dependent granulocytic markers, i.e., granulocyte-colony-stimulating factor receptor mRNA and leukocyte alkaline phosphatase. However, IFN(alpha) does not have any effects on the retinoid-dependent regulation of the myeloid surface markers CD11b and CD33. The IFN-dependent increase in RAR(alpha) levels and the enhancing effect of the cytokine on retinoid-dependent granulocytic markers expression may be a characteristic of PML-RAR positive cells, since the phenomena are not observed in HL-60 promyelocytes. Interferons as well as retinoids inhibit the growth of NB4 cells, although the 2 classes of compounds do not significantly interact in terms of anti-proliferative activity. These results suggest the possible use of combinations between IFNs and retinoic acid in the cyto-differentiating treatment of APL patients.