Lyophilization of bovine ribonuclease A (RNase A; Sigma, type XII-A) from 40% acetic acid solutions leads to the formation of approximately 14 aggregated species that can be separated by ion-exchange chromatography. Several aggregates were identified, including two variously deamidated dimeric subspecies, two distinct trimeric and two distinct tetrameric RNase A conformers, besides the two forms of dimer characterized previously [Gotte, G. & Libonati, M. (1998) Two different forms of aggregated dimers of ribonuclease A. Biochim. Biophys. Acta 1386, 106-112]. We also have possible evidence for the existence of two forms of pentameric RNase A. The two forms of trimers and tetramers are characterized by: (a) slightly different gel filtration patterns; (b) different retention times in ion-exchange chromatography; and (c) different mobilities in cathodic gel electrophoresis under nondenaturing conditions. Therefore, they appear to have distinct structural organizations responsible for a different availability of their positively charged amino acid residues. All RNase A oligomers, in particular the two distinct trimeric and tetrameric conformers, degrade poly(A).poly(U), viral double-stranded RNA and polyadenylate with a catalytic efficiency that is in general higher for the more basic species. On the contrary, the activity of the RNase A oligomers, from dimer to pentamer, on yeast RNA and poly(C) (Kunitz assay) is lower than that of monomeric RNase A.