Based on predictions of the structure of proteinase 3C of poliovirus, mutations have been made at residues that are supposed to constitute the catalytic triad. Wild-type and mutant 3C were expressed in Escherichia coli, purified to homogeneity, and characterized by the ability to cleave a synthetic peptide substrate or an in vitro translated polypeptide consisting of part of the polyprotein of poliovirus. Additionally, the ability of autocatalytic processing of a precursor harboring wild-type or mutant 3C sequences was tested. Single substitutions of the residues His-40, Glu-71, and Cys-147 by Tyr, Gln, and Ser, respectively, resulted in an inactive enzyme. Replacement of Asp-85 by Asn resulted in an enzyme that was as active as wild-type enzyme in trans cleavage assays but whose autoprocessing ability was impaired. Our results are consistent with the proposal that residues His-40, Glu-71, and Cys-147 constitute the catalytic triad of poliovirus 3C proteinase. Furthermore, residue Asp-85 is not required for proper proteolytic activity despite being highly conserved between different picornaviruses. This indicates that Asp-85 might be involved in a different function of 3C.