Human colon sialidase has been characterized, and its activity levels in normal mucosa and colonic adenocarcinoma have been determined. Sialidase activity was maximal at pH 5.5. and was unstable with storage at 4 and -20 °C. The bulk of activity was pelletassociated. and could not be released with triton X-100 or 3-([3-cholamidopropyl]- dimethylammonio)-l-propanesulfonate. Using 2’-(4-methylumbelliferyl)α-D-N-acelylneuraminic acid as substrate, the K(m) and V(max) values were estimated to be 0.140 mmol/1 and 63 mU/g. respectively. Furthermore, an inhibition by substrate concentrations above 1.5 mmol/1 was detected. Neuraminic acid caused a competitive inhibition with a K(1) of 3.5 mmol/1. A statistically significant increase (p< 0.001) in the sialidase specific activity was found in primary colonic adenocarcinoma (104.20±8.00 mU/g) compared to that of the normal mucosa (72.50±7.67 mU/g).