In searching for alternative ways to reduce or inhibit amyloid formation, we have studied this process using hen egg white lysozyme (HEWL) in the presence of a low concentration of protic ionic liquids. The ionic liquids were. synthesized in a combinatorial fashion maintaining the cationic part (tetramethylguanidinium) with alteration of the anionic component of each compound tested. It was observed that one of these compounds (tetramethylguanidinium acetate) inhibited amyloid formation of HEWL in vitro by nearly 50%. Examination under transmission electron microscopy confirmed the fibril inhibition, and fibrils were observed to be morphologically thinner. To investigate the mechanism of inhibition, intrinsic fluorescence, ANS binding, and circular dichroism analyses were performed. These analyses indicated that the native structure of HEWL was maintained in the presence of the ionic liquid. Performing native PAGE and nondenaturing agarose electrophoresis, it became evident that some of the intermediate oligomers were not converted to protofibrils and that the oligomers were trapped in more stable conformations. Additionally, it was observed that this inhibitory effect was related to the ionic liquid itself and not the solvated ions. It also became evident that the carboxyl functional group was important in the inhibition. The size of the anions and kosmotropicity did not play significant roles in the fibril inhibition.