A novel cyclotetrapeptide cyclo(-L-Pro-L-Tyr-L-Pro-L-Val-) was isolated from the lactic bacterium Lactobacillus helveticus. The structure was determined by spectroscopic means and confirmed by acid hydrolysis of the peptide and sequence analyses of the resulting linear peptides.Tyrosinase (phenol oxidase) [EC 1.14.18.11 is known to be a key enzyme for melanin biosynthesis in plants, microorganisms and mammalian cells, and also known to be as a copper-containing enzyme13). Many compounds, such as kojic acid and arbutin, have been reported as tyrosinase inhibitors 4*5). During our continuous screening of the inhibitors produced by lactic bacteria, we found the potent activity in Lactobacillus helveticus. This paper describes the isolation, structure, and activity of a novel tyrosinase inhibitor from the bacterium.