It was shown that H1 type histones possess anticancer activity and could be utilized in therapy of acute myeloid leukemia. We developed an experimental pilot-scale technology of the recombinant histone H1.3 variant production. The downstream process includes acidic extraction of the target protein from the culture broth, ion-exchange, reversed-phase and size-exclusion chromatography, and freeze-drying. The accent was made on the reduction of bacterial endotoxin contamination of the active pharmaceutical ingredient. Highly efficient downstream strategy of the target protein depyrogenation is discussed in the paper. The developed technology allows the production of the protein with high yield (approx. 110 g per 1000 L of the culture broth) and of high purity (estimated productivity is 75-100 g/month). The activity and purity of the active pharmaceutical ingredients produced for clinical trials were confirmed by different tests including ultra performance liquid chromatography, size exclusion chromatography, ELISA, SDS-PAGE, gel-clot LAL-test. Clinical trials were started in the Russian Federation. (C) 2011 Elsevier Ltd. All rights reserved.