As a potential measure of active biomass in an automated and continuous system, enzyme activities of activated sludge were sought that would allow a rapid and accurate determination. Using p-nitrophenylphosphate, bis-p-nitrophenylphosphate, p-nitrophenyl-α- and β-D-gluco- or galactopyranosides, and the p-nitroanilides of L-alanine, L-leucine, L-lysine, L-glutamic acid and L-phenylalanine, we could establish that the corresponding phosphatase, cyclic phosphodiesterase, glycosidase and aminopeptidase activities can be conveniently tested with diluted activated sludge within 10–20 min incubation at 30°C. The reactions were linear with time and concentration of activated sludge. The specific activities were in the range from 1 to 50 nmol substrate cleaved/min/mg protein at 30°C. They were diminished by starvation or poisoning with zinc powder, ZnCl2, HgCl2 or KCN. The enzymes were over 95% sedimented together with the flocs. Comparable activities were found in sludges from three independent sewage works in the Munich area. At the same time, dehydrogenase activities and ATP contents were investigated under several conditions.