"Zero-length" dimers of ribonuclease A, a novel type of dimers formed by two RNase A molecules bound to each other through a zero-length amide bond [Simons, B. L., et al. (2007) Proteins 66, 183-195], were further characterized and tested for their possible in vitro cytotoxic activity. Results obtained are the following. Besides dimers, also trimers and higher oligomers could be identified among the products of the covalently linking reaction, and the "zero-length" dimers prepared by us appear not to be a unique species. The product was indeed heterogeneous, and results obtained with two RNase A mutants, E9A and K66A, indicated that amino and carboxyl groups others than those belonging to Lys66 and Glu9 are involved in the amide bond. As for their functional properties, the "zero-length" dimers degrade poly(A).poly(U) (dsRNA) with an activity that increases with the increase of the oligomer's basicity and yeast RNA (ssRNA) with an activity that instead decreases with the increase of oligomer's basicity, which is in agreement with previous data. No cytotoxicity of the RNase A "zero-length" dimers could be evidenced in assays performed with various tumor cells lines; the dimers, instead, become cytotoxic if cationized by conjugation with polyethylenimine (PEI) [Futami et al. (2005) J. Biosci. Bioengin. 99, 95-103]. However, PEI derivatives of RNase A "zero-length" dimers and PEI derivatives of native RNase A resulted to be equally cytotoxic. In other words, protein "dimericity" does not play any role in this case. Moreover, the acquired cytotoxicity does not seem to be specific for tumor cells: PEI-cationized native RNase A was also cytotoxic toward human monocytes.