||The ongoing collaboration between IFREMER-Centre de Brest and NTH at the University of Trondheim (contract N° 91/2 430 423/C: Etude du rôle de l'activité enzymatique des entérobactéries), has resulted in increased knowledge of enzymatic activities of polluted coastal waters of France and Norway. Measurement of enzyme activities (methylumbelliferyl-B-galactosidase (MUGalase) and methylumbelliferyl-B-glucuronidase (MUGluase)) of fecal coliform bacteria (FC) are presently used for rapid detection of FC bacteria in water. During the investigations the question of influence by environmental conditions has been addressed. Among these is the effect of lactose on MUGalase and MUGluase-activity. The presence of lactose could be important for enzyme activity of cells in the environment as well as during analysis of enzyme activity. Sarhan and Foster (6) showed that lactose prevented the activity of MUGluase assessed as fluorescing colonies on solid media after 7.5 hours of incubation. The mechanism of this inhibition was not clear and several possibilities were suggested, e.g acid production by fermentation of sugars which may inhibite enzyme production or enzyme activity during growth.