Histones normally exist in the cell nuclei, but are also released outside to the lumen forming so-called extracellular histones. Extracellular histones pose strong cytotoxicities to cells, particularly in the disease pathogenesis and progression of sepsis, acute lung injuries and rheumatoid arthritis. The quantification of extracellular histone levels in the patients’ plasma demands high quality histone standard. Otherwise, circulating histone levels will not be usable for the disease stage classification and evaluation of drug dosage. In laboratories, quantification of extracellular histones is used for exploring the mechanism of histone toxicity. In the present study, we expressed denatured histone H3 and H4 monomers in E. coli. After purification with affinity column, the refolded H3 and H4 monomers, as well as H3/H4 complex, could be obtained by either gradient dilution or dialysis. We found that the H3/H4 complex was the most stable form of recombinant histones. Treatment of HUVECs with 50 μg/mL recombinant H3/H4 complex induced about 80% cell death, equivalent to the treatment with 200 μg/mL calf thymus histones. Such cytotoxicity could be relieved dosedependently with human serum albumin from 0.625～10 mg/mL, which was similar to the situation with natural H3/H4 complex. Therefore, our recombinant H3/H4 complex was usable as a quantification standard for the study of histone-induced cytotoxicities.