A second generation mutant of dimeric human pancreas RNase (HHP2-RNase), was obtained by a single residue mutation (Glu(111)-->Gly) of the previously described dimeric human pancreas RNase variant (HHP-RNase). HHP2-RNase was found to be a highly specific antitumour agent, with an enhanced cytotoxic activity compared with HHP-RNase. The structural and functional requisites of the antitumour action of HHP2-RNase were investigated and compared with those of other dimeric antitumour RNases. The stability of the dimeric structure, i.e. the resistance of human dimeric RNase variants to reductive cleavage of the two intersubunit disulphide bonds that bridge the subunits, was determined to be an essential feature of antitumour dimeric RNases. The stability of the dimeric structure is in turn responsible for the resistance to inhibition by the cytosolic RNase inhibitor (cRI). Both the stability of the dimeric structure and the resistance to cRI inhibition appeared to be highly enhanced by an RNase substrate. This suggests a possible role for RNA in the amplification of the antitumour potential of dimeric RNases.