Dye-ligand chromatography has been used successfully for the purification of extracellular thermostable proteinases from thermophilic Bacillus and Thermus cultures. Single-step purification factors of up to 115-fold (for Thermus protease) and 2195-fold (for Bacillus protease) were obtained. Elution studies suggested that the mode of binding involved the enzyme active sites. The method was readily scalable to 600 1 volume.