All trans-retinoic acid (ATRA) can induce granulocytic differentiation both in vitro and in vivo, and its activity is mediated by the retinoic acid receptor-alpha (RAR-alpha). In the present study, we evaluated the ability of this inducer in HL-60 cells, to stimulate simultaneously granulocytic differentiation and the expression of the cytokines interleukin-1 alpha (IL-1 alpha), IL-1 beta, IL-3, IL-6, tumor necrosis factor-alpha (TNF-alpha), and stem cell factor (SCF). The level of expression of these cytokines in ATRA-treated HL-60 cells was compared with that observed in normal and lipopolysaccharide (LPS)-treated peripheral granulocytes. The results indicate that the expression of these cytokines is enhanced during differentiation so that the pattern observed in ATRA-treated HL-60 cells is close to that of LPS-stimulated normal granulocytes. In addition, tetra phorbol acetate (TPA)-treated HL-60 cells express several of the above listed cytokines. It is concluded that ATRA not only induces granulocytic differentiation of HL-60 cells, but also activation of these terminally differentiated cells. The activating cytokine expression in these cells appears related to the progress of the differentiation program induced by ATRA since normal granulocytes do not respond to this inducer by activation of the expression of these genes. Furthermore, the cytokine activation is a specific effect of ATRA, since DMSO does not have any stimulatory effect.