Solutions of DL-phenylalanine-NCA in eight different solvents were thermostated at 20 degrees C for 4 days (and in two cases at 60 degrees C) without addition of initiators. Spontaneous polymerizations were observed in dimethylformamide (DMF), in N-methylpyrrolidone (NMP), and in dimethyl sulfoxide (DMSO). In dimethyl sulfoxide also sarcosine-NCA, L-alanine-NCA, D,L-leucine-NCA, and DL-valine-NCA underwent spontaneous polymerizations. However, in N-methylpyrrolidone, only sarcosine-NCA and L-alanine-NCA polymerized. The products of all these solvent-induced polymerizations were identified by MALDI-TOF mass spectrometry as cyclic oligo- and polypeptides. This fact and the finding that only the most nucleophilic solvents induce spontaneous polymerizations indicate a zwitterionic polymerization mechanism.