Poliovirus protease 3C is a cysteine enzyme that is essential for the processing of the viral precursor polyprotein containing structural proteins and enzymes, including the protease itself. We have constructed the plasmid pSD/PV3C which produced protease 3C as inclusion bodies when expressed in Escherichia coli. In addition to the full-length protease, a truncated form was also generated, starting from an internal initiation site (Met-27). The enzyme was renatured by dilution of a 6 M guanidinium chloride solution of the inclusion bodies, and the proteins were precipitated from the diluted solution with ammonium sulphate. By extracting the precipitate with a buffer solution, the full-length enzyme could be completely separated from its N-terminally truncated form. Size-exclusion chromatography of the extracted protease 3C resulted in an active enzyme which appeared homogeneous by SDS/PAGE. For measuring the activity of the protease, a spectrofluorimetric method was devised to monitor the hydrolysis continuously, which is simpler and more precise than the h.p.l.c. technique used previously.