A cell density monitor, comprising parts of a liquid chromatography system, was adapted for on-line real-time measurement of bacterial cell density up to 75 g l(-1). The device dilutes the sample containing cells and measures the absorbancy at 280 nm. This detector gives an excellent correlation with off-line measurements, good reproducibility, a low limit of detection (3.5 mg l(-1)), and becomes a low cost alternative for fermentation monitoring in small stirred bioreactors.