We demonstrated that tumor necrosis factor-alpha (TNF-alpha) rapidly and markedly enhances p21 gene and protein expression prior to monocytic differentiation and apoptosis in p53-null HL-60 cells. TNF-alpha induced early small and delayed large peaks of apoptosis at 6 and 48 h of incubation, respectively. At 24 h of incubation, apparent monocytic differentiation of the cells was noted. Down-regulation of c-myc and c-myb and G0/G1 arrest were observed at 6-12 and 36 h, respectively. Actinomycin D markedly inhibited TNF-alpha-induced p21 mRNA expression, suggesting that the p21 gene is induced at the transcriptional level. We confirmed that TNF-alpha induces p53-independent apoptosis in HL-60 cells, which accompanies monocytic differentiation.