Bacillus licheniformis (DSM 641) was cultivated on complex medium in batch and fed-batch operations in a 20-l working volume stirred tank reactor. The medium composition (maltose, glucose, sucrose, fructose, ammonia, phosphate) and O2 and CO2 in the off-gas were monitored on-line; pH, pO2, turbidity, culture fluorescence were monitored in situ; optical density, concentrations of sugars, amino acids, phosphate, proteins, DNA, protease activity and total solids content were monitored off-line. Problems of on-line sampling, cell concentration monitoring, and culture fluorescence measurements and the influence of medium components on the enzyme productivity are discussed. Close relationships between variations of pH, pO2, O2 transfer rate and CO2 production rate on the one hand and cell mass and fluorescence intensity on the other were demonstrated in batch and in fed-batch cultures. Using suitable cultivation conditions, alkaline protease with high volume activity [15300 units (U)/ml] and specific activity (510 U/mg) was produced. By replacing the complex medium with a semisynthetic one, the volumetric activity was reduced by a factor of ten (to 1650 U/ml), but the specific productivity by a factor of only two (to 210 U/ml).