The recent paper of Sherr and Sherr on detecting low-affinity beta-glucosaminidase activity in several marine microbes extends current knowledge about hydrolytic enzyme activities in natural aquatic systems. However, their conclusions regarding the whole-cell assay with MUF-N-acetyl-beta-D-glucosaminide (MUF-[GlcNAc]) cannot be accepted. First, we explicitly demonstrate a strong correlation between extracellular activities of the high-affinity enzymes and grazing rates of bacterivorous protists. Therefore, the assay can still be recommended for the estimation of total protistan grazing on prokaryotic picoplankton. Second, the ability of many aquatic organisms to produce enzymes which cleave fluorogenic substrates, such as MUF-[GlcNAc] and/or MUF-beta-D-N,N’,N"-triacetylchitotriose (MUF-[GlcNAc](3)), has been well-documented during the last decade. Thus, neither of the two substrates may be considered as exclusively specific for targeting either lysozymes or beta-N-acetylhexosaminidases. (C) 2000 Federation of European Microbiological Societies. Published by Elsevier Science B.V. All rights reserved.