The assay on microbial dehydrogenase activity by using TTC was investigated. The experimental results are summarized as follows: 1. The TF production was affected by dissolved oxygen. Dissolved oxygen in samples could be effectively removed by adding $Na_{2}SO_{3}$ as reductant prior to incubation. 2. The toxic effect on TF formation was observed at up to 0.1% of final TTC concentration when VSS concentration was 2.7g/l. As the VSS concentration increased the effect decreased. 3. It was clearly revealed that the TF generation rate decreased with increasing VSS concentration. The dilution of sludge could be applicable when VSS level is below 3.5g/l. 4. The TF production of endogenous phase (De) could be used as a measure for the viable fraction of sludge over various sludge ages. The additional TF production (Ds) due to extracellular substrate responded to De, not to VSS concentration.