A novel approach for indirect biomass detn. in case of non-const. specific metabolic rates is reported. Specific glucose consumption rate was detd. during a first calibration culture for both free suspended and immobilized cells. It was expressed as a linear function of the limiting substrate, i. e. glucose for suspension and oxygen for immobilized cell cultures. For validation, these time-independent models were applied on a second culture presenting a different inoculum cell d. Online monitoring of the limiting substrate enabled a continuous detn. of the specific metabolic rate and thus online cell no. prediction, which was verified with an off-line cell counting technique. [on SciFinder (R)]