Cyclic oligomers of 6-aminohexanoate were fractionated from waste materials obtained from a nylon-6 factory by Sephadex LH-20 gel filtration column chromatography, and two major fractions, a cyclic tetramer and cyclic dimer, were obtained. The 6-aminohexanoate-oligomer hydrolase (EIII) from Flavobacterium sp. K172 was active toward the cyclic tetramer, but not toward the cyclic dimer. The M(r) of the EIII enzyme was estimated to be 93,000 by high performance gel filtration chromatography (TSK-G3000SW), while that of the polypeptide as deduced from the nucleotide sequence was calculated to be 36,902, suggesting that the EIII enzyme has a dimeric or trimeric structure. The enzyme was stable between the pH 6.5 and 11 with an optimum pH of 7.0. The enzyme activity was stable up to 45 degrees C with an optimal temperature of 40 degrees C.