A simple affinity chromatography procedure for specific isolation of serine proteases is described. The procedure was tested using enzymes from five microbial and one plant source. Feather keratin, covalently bound to controlled-pore glass, was the support and magnesium chloride was used in the elution buffers instead of zinc chloride. This enabled one-step isolation of serine proteases present in the biological materials used. The small (15 cm x 1 cm) controlled-pore keratin-glass column allowed high flow rates and protected the proteases from autodigestion during the chromatography process. The serine proteases were eluted from the column with good recovery (40-84-6%) and a purification efficiency between 5 and 7. The purified proteases were homogeneous by polyacrylamide gel electrophoresis.