All-trans retinoic acid (ATRA) has been shown to induce differentiation of human acute promyelocytic leukaemia (APL) cells and eventual elimination of the malignant clone. Matrix metalloproteinase-9 (MMP-9) is produced by neutrophils and its expression appears to be linked with myeloid cell differentiation. We investigated effects of ATRA on MMP expression in two human myeloid leukaemia cell lines, PL-21 and NB4. Both cells could differentiate into neutrophils after exposure to ATRA. Both the activity and antigen levels of MMP-9 were much higher in NB4 cells than in PL-21 cells. Stimulation with ATRA significantly increased MMP-9 levels approximately three- to fivefold in both PL-21 and NB4-conditioned media. MMP-9 mRNA levels increased in ATRA-treated cells and was almost in parallel with the increase in MMP-9 activity, suggesting that ATRA induced MMP-9 by activating its gene expression. ATRA can induce interleukin 8 (IL-8) in APL cells. IL-8, chemokine for neutrophils and a potent inducer of MMP-9, was also induced by ATRA in PL-21 cells. However, recombinant IL-8 did not induce MMP-9 expression. In addition, a neutralizing antibody against IL-8 did not inhibit ATRA-induced MMP-9 expression in either cell type. These observations suggest that ATRA can induce both MMP-9 and IL-8, but IL-8 is not involved in ATRA-induced MMP-9 expression. As MMP-9 can truncate and activate IL-8, simultaneous induction of MMP-9 and IL-8 by ATRA could activate leucocytes excessively, causing the hyper-inflammatory events in retinoic acid syndrome.