A fluoride sensitive (pF), buffer capacity insensitive electrolyte isolator semiconductor capacitor (pF-EIS-CAP) chip was used as a transducer for biosensors with co-immobilized enzymes. Hydrogen peroxide forms fluoride ions in stoichometric amounts in the presence of p-fluoraniline and peroxidase (POD). This was measured by the pF-EIS-CAP-sensor. The combination of this sensor with various oxidases allows the measurement of the concentration of several analytes (Beyer et al., Biosensors & Bioelectronics, 1994, 9, 17-21; Menzel et al., Analyt. Chim. Acta, 1995, 317, 259-264).Sugar syrup (glucose, maltose, maltotriose and other oligosaccharide) concentrations were analysed with co-immobilized beta-amylase, amyloglucosidase, mutarotase, glucose oxidase and peroxidase. The glucose concentration was analysed with co-immobilized glucose oxidase and peroxidase, the ethanol concentration with co-immobilized alcohol oxidase and peroxidase and the phosphate concentration by co-immobilized nucleoside phosphorylase, xanthin oxidase and peroxidase. These (Bio-pF-EIS-CAP) sensors were integrated in flow injection analysis systems. The medium components were monitored on-line during Saccharomyces cerevisiae, Acremonium chrysogenum and recombinant baby hamster kidney (rBHK) cell cultivations. The agreement between the off-and on-line concentrations of the medium components was within 10%.