The co-chaperonin GroES (Hsp10) works with chaperonin GroEL (Hsp60) to facilitate the folding reactions of various substrate proteins. Upon forming a specific disordered state in guanidine hydrochloride, GroES is able to self-assemble into amyloid fibrils similar to those observed in various neurodegenerative diseases. GroES therefore is a suitable model system to understand the mechanism of amyloid fibril formation. Here, we determined the cytotoxicity of intermediate GroES species formed during fibrillation. We found that neuronal cell death was provoked by soluble intermediate aggregates of GroES, rather than mature fibrils. The data suggest that amyloid fibril formation and its associated toxicity toward cell might be an inherent property of proteins irrespective of their correlation with specific diseases. Furthermore, with the presence of anthocyanins that are abundant in bilberry, we could inhibit both fibril formation and the toxicity of intermediates. Addition of bilberry anthocyanins dissolved the toxic intermediates and fibrils, and the toxicity of the intermediates was thus neutralized. Our results suggest that anthocyanins may display a general and potent inhibitory effect on the amyloid fibril formation of various conformational disease-causing proteins.