Abstract: Based on the element balances and bioreactor material conservation of a biochemical reaction, adaptive estimation theory was employed to eliminate white noises during the estimation of cell concentration in spiramycin fermentation processes. The scheme precluded the estimation problems introduced by the complexity and uncertainty of cell growth models. In addition, the relation between the total nucleic acid concentration and the cell concentration was investigated. During spiramycin fermentation, which lasted 100 hours, the estimation values of cell concentration agreed quite well with the experimental results.