In order to study the role of PML-RARalpha fusion gene and its expression product during apoptotic process in NB4 cells induced by arsenic trisulfide (As(2)S(3)), the apoptotic effects of NB4 cells were observed by cell morphology, flow cytometry and DNA electrophoresis. The change of PML-RARalpha fusion gene and its expression product were also assayed by chromosomal G banding, RT-PCR and Western blot. The results showed that arsenic trisulfide induced apoptosis of NB4 cells, during this process, PML-RARalpha fusion gene had no significant changes, but the expression of PML-RARalpha fusion protein and wild-type RARalpha were all reduced. It is concluded that arsenic trisulfide can induce apoptosis of NB4 cells, the degradation of PML-RARalpha fusion protein and wild-type RARalpha may play an important role during apoptotic process.