In this report we show a strong synergistic interaction between granulocyte colony-stimulating factor (G-CSF) and all-trans retinoic acid (ATRA) on the expression of leukocyte alkaline phosphatase (LAP) in freshly isolated acute promyelocytic leukemia (APL) blasts as well as in NB40 and HL-60 cell lines. The strong synergism observed in these cell types was not evident in two acute leukemia cell lines (K562 and GF-D8), in normal granulocytes, and in monocytes. In freshly isolated leukocytes derived from chronic myelogenous leukemia (CML), in the stable phase of the disease, a weaker interaction between ATRA and G- CSF was documented. The cross-talk between the cytokine and the retinoid was studied in detail in NB4, an immortalized APL leukemia cell line, retaining the 15′17 chromosomal translocation involving the retinoic acid receptor type alpha. The treatment of NB4 cells with G- CSF alone or ATRA alone leads to no increase and to minor induction in LAP activity, respectively. If the cells are treated with the two compounds simultaneously, a dramatic elevation of LAP is observed after 4 days. The synergism between G-CSF and ATRA is evident at concentrations of the retinoid between 10(-7) and 10(-5) mol/L and at concentrations of the cytokine between 1 and 10 ng/mL. The simultaneous presence of the two compounds is necessary to obtain maximal increase of LAP activity and the effect is cell density-dependent. Synergism is specific for G-CSF, and it is not observed with other cytokines and functional inducers of the granulocyte. The augmentation of LAP activity is the consequence of an increased transcriptional rate of the liver/bone/kidney-type (L/B/K-type) alkaline phosphatase gene, as determined by Northern blotting and nuclear run-on analysis using specific cDNA probes. Only one of the two possible alternatively spliced forms of L/B/K-type alkaline phosphatase transcript is detected in NB4 cells after stimulation with G-CSF and ATRA. This mRNA form, which is the one observed in normal polymorphonuclear leukocytes, contains the most upstream leader exon. In NB4 cells, ATRA induces G- CSF, alpha, and beta retinoic acid receptor transcripts, whereas G-CSF has minor effects on the expression of these mRNAs.