1 We have examined the effect of the histone deacetylase inhibitors apicidin, trichostatin A (TSA) and n-butyrate on the histone acetylation and the differentiation of human eosinophilic leukemia HL-60 clone 15 cells into eosinophils. 2 Viability of the cells incubated with apicidin (100 nm), TSA (30 nm) or n-butyrate (500 microm) did not change significantly, but higher concentrations of apicidin (> or =300 nm) or TSA (> or =100 nm) decreased the viability when examined at day 1. 3 Apicidin (100 nm) as well as n-butyrate (500 microm) induced continuous acetylations of histone H4 and lysine14 residue on histone H3, while TSA (30 nm) induced transient acetylations. 4 After 6 days incubation, eosinophilic cells stained by Luxol-fast-blue were generated by apicidin (100 nm) and n-butyrate (500 microm) but not by TSA (30 nm). Other markers for differentiation into eosinophils such as changes in intracellular structure, and expressions of integrin beta7 and major basic protein, and the inhibition of cell proliferation were also induced by apicidin and n-butyrate but not by TSA. 5 Continuous acetylation of histone H4 achieved by repeated treatment with TSA (30 nm) at an interval of 12 h for more than three times induced such changes when examined on day 6. In addition, the induction was impaired by shortening the period of incubation with apicidin (100 nm) or n-butyrate (500 microm). 6 CCAAT/enhancer binding protein was continuously activated by apicidin (100 nm) and n-butyrate (500 microm), but was transiently activated by TSA (30 nm). 7 These findings suggest that the continuous acetylation of histones H3 and H4 is necessary for the differentiation of HL-60 clone 15 cells into eosinophils.