A new method for obtaining HIV-I protease was suggested. Fusion proteins composed of the N-terminal fragment of human gamma-interferon and HIV-I protease connected with (Asp)(4)Lys (protein I) or Asp-Pro (protein II) linkers were expressed in Escherichia coli cells. The fusion proteins were produced as insoluble inclusion bodies in the 20% yield of total cell protein. Protein I was cleaved by enterokinase. The solubility of protein I was increased by treating with Na-sulfite/Na-tetrathionate under denaturing conditions. Optimal conditions for efficient acidic hydrolysis of protein II at Asp-Pro bond were found. The hydrolysis products were separated by reversed-phase FPLC. The amount of tryptophan and cysteine residues in the enzyme obtained was estimated. The activity of HIV-I protease was determined using the chromogenic peptide AlaArgVal NleNphGluAlaNleNH(2) and a high-mol-wt substrate consisting of beta-galactosidase and a fragment of gag proteins, including p17-p24 processing site.