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Cationization of a protein is considered to be a powerful strategy for internalizing a functional protein into cells. Cationized proteins appear to adsorb to the cell surface by electrostatic interactions, then enter the cell in a receptor- and transporter-independent fashion. Thus, in principle, all cell types appear to take up cationized proteins. Since ribonucleases (RNases) have a latent cytotoxic potential, cationized RNases could be useful cancer chemotherapeutics. In this study, we investigated the effect of the degree of cationization on the cytotoxicity of RNase A by modifying carboxyl groups with ethyl-enediamine. We found that there is an optimum degree of modification for cytotoxicity, in which 5 to 7 out of 11 carboxyl groups in RNase A are modified, toward MCF-7 and 3T3-SV-40 cells. More interestingly, the cytotoxicity of cationized RNase As correlates well with the value of [RNase activity] x [estimated concentration of RNase free from RNase inhibitor], mimicking the practical enzymatic activity of cationized RNase As in cytosoL The results indicate that cationization of a protein to an optimum level is important for maintaining protein function in the cytosoL Sophisticated protein cationization techniques will help to advance protein transduction technology. [ABSTRACT FROM AUTHOR]