The aim of this paper is to determine the efficiency of bioleaching of arsenic in realgar, a Chinese mineral drug, using pure cultures of Acidithiobacillus ferrooxidans or Acidithiobacillus thiooxidans and a mixed culture of A. ferrooxidans and A. thiooxidans. The experiments were carried out in shaker flasks, at 150 rpm, 30 degrees C at a culture pH of 1.80. To investigate the mechanism of the bioleaching in realgar, media with and without ferrous iron were chosen for the experiments. The results showed that the leaching rate of arsenic in realgar after 20 days was higher (43%) in A. ferrooxidans cultures with ferrous iron compared to cultures without ferrous iron (10%), and the leaching rate of A. thiooxidans cultures only increased from 21% to 23% in the presence of ferrous iron. The leaching rate of arsenic in mixed culture with ferrous iron was greatly enhanced from 16% to 56%, indicating that bioleaching in mixed culture is preferable for the dissolution of realgar. (c) 2007 Elsevier Ltd. All rights reserved.