The PROIMI-2 prototype turbidity-meter for measuring cell concentrations in stirred tanks, has been designed and constructed using theories based on kinetic laws and the general principles of cell growth. Laboratory assays were carried out with batch cultures of Bacillus amyloliquefaciens, Zymomonas mobilis and Saccharomyces cerevisae. The signals showed a good correlation with optical density and biomass. In the latter case up to 4.0 per dm(3) expressed as dry weight could be monitored successfully.