Dimeric seminal RNase presents the singular case of a dimer with access at equilibrium to two conformations: one in which the subunits exchange, or swap, their NH(2)-terminal arms; the other with no exchange. Thus a continuous unfolding/refolding of structural elements into two alternative conformations takes place in the native protein at equilibrium. The phenomenon was investigated by kinetic and mass spectrometric analyses of the effects of trypsin on the native protein, on its isolated quaternary forms, as well as on a monomeric derivative of the protein and on homologous dimeric RNase A. The kinetics of tryptic action on the protein forms and on the protein derivatives, as well as the location of the tryptic cleavage sites, and their chronological sequence, led to the identification of relevant interconversion intermediates, to the description of a model for the interconversion process, and to a hypothesis for the unique phenomenon of the dual quaternary conformation of seminal RNase.