Retinoic acid-mediated differentiation of HL60 cells is associated with an alteration of chromatin structure that maps to protein-binding sequences within intron I of the c-myc gene and with down-regulation of c-myc expression. By using HeLa cell extracts, we previously identified two polypeptides, designated MIBP1 (for Myc-intron-binding peptide) and RFX1, that interact in vivo and bind to the intron I element; we showed that tandem repeats of an MIBP1/RFX1-binding site can exhibit silencer activity on a heterologous promoter. Here we demonstrate that p160 MIBP1 and p130 RFX1 are absent from undifferentiated HL60 cells. In addition, we show that treatment with retinoic acid induces both MIBP1 and RFX1 protein, as well as their DNA-binding activity, upon granulocytic differentiation of HL60 cells, with a gel mobility pattern identical to that of HeLa cells. In the absence of p160 MIBP1 and p130 RFX1, we observed that the altered gel mobility-shift pattern detected in undifferentiated HL60 cells reflects the binding of two novel polypeptides, p30 and p97, that can be cross-linked to the same recognition intron sequence. We also show that the time course of MIBP1 and RFX1 induction is inversely correlated with the down-regulation of c-myc levels during the treatment of HL60 cells with retinoic acid.